In the second half of Chapter Five, Darwin discusses the differing degrees of variability found in related organisms. He claims that features exaggerated in one species compared to others of the same genera are more likely to vary within that species, than the same features found in entire genera for which none have the features in question exaggerated. In other words, he says (albeit in a great many more words) that evolution has not yet stopped playing with highly characteristic features: the effects of evolution are still visible after speciation. The features in question will eventually stabilise, but only after enough time has passed to allow the new species to have become new genera. This is an interesting idea, but I am not sure how well supported it is by modern data, and I have yet to look. It would be interesting to have this confirmed.
A related idea is that secondary sexual characteristics -- generally those characteristics visibly distinguishing male from female, but properly those characteristics not directly linked with reproduction (and represented as such by Darwin) -- vary within species more than those characteristics common to both sexes. Again, I am not familiar with the details here, but taking Darwin at his word, one might find some sense in this which Darwin himself seems not to have noted. Here I mean the fact that secondary sexual characteristics include those which individuals (typically but not always females) use in choosing from multiple suitors (typically but not always males) are in fact often species identifiers as well. A bird's song, for instance, carries many messages, which could include that the singer is male, well-fed, and looking for a mate -- but also, and always (at least when signalling specifically to members of the same species), that the singer is of a given species. While aspects of the song might vary from one individual to another (and they invariably will), any significant departures from the cues used to identify species will result in that individual failing to attract the attention of either mates or rivals. What features of a call characterise a species differs from one species to the next; and in the process of speciation, a consensus might form around a particular variant of some feature which previously had no species-descriptive function, and over time this consensus might result in that variant of that feature becoming "fixed" for the population. Ultimately, if the population becomes a distinct species, that feature would become a descriptor of the new species. In other words, the variation of secondary sexual characteristics (of which a bird's mating song is one) is itself one possible engine driving the generation of new species.
The remainder of the chapter features Darwin blindly groping about to connect his theory to his (mis)understanding of genetics. Specifically, he spends a good bit of it discussing reversions, which he rightly supposes to be preserved somehow in the makeup of individuals far removed from their ancestors displaying the features to which they have reverted. There really is not very much to say about this from a modern perspective, or even from a classical-genetics perspective: one may continue to remain completely ignorant of DNA and still be able to follow Mendel's and his successors' explanations. What Darwin does do well here, however, is point out how unrealistic the explanations for his examples must be in the context of what he calls special creation, the notion that each species was created separately. This was, of course, a major purpose to his work -- to establish evolution as a unifying principle in biology, and to show the uselessness of the competing theory of special creation. Much of his work is overkill here, but then the theory of evolution required a great deal of support to replace what had been asserted as truth for literally millennia before.
Tuesday, 10 February 2009
Monday, 9 February 2009
The Origin, Chapter Five, Part One
This chapter is entitled "Laws of Variation", which implies that it will discuss genetics, the subject which Darwin famously got wrong. That having been said, Darwin is also very forthright in admitting his ignorance, that his understanding of genetics is in effect a hunch. Surprisingly, he actually noticed the proportions expected of dominant allele inheritance that Mendel pursued further to propose the first viable model of particulate inheritance. Had he only investigated that further himself! But he did not; and it is a testament to his model of evolution that it did not matter to his main point. Darwin assumed a "blending" model of inheritance, in which -- on average -- the character states of any set of offspring will represent the mean between their parents. This does not happen with particulate inheritance, in which a state is either present or not, and can persist for many generations in spite of individuals having it mating with those having its corresponding opposite state. How could Darwin's model work to preserve useful mutations? Remember that I said that the character states of individuals were on average the mean between those of their parents: variation will always occur in Darwin's model, and may offset the tendency for extreme characters to "average out" into the less remarkable norm.
To put it another way, suppose that a natural amount of variation exists in a population, such that some trait can be measured as being anywhere from x to -x, and all values have an even distribution. The average value of the trait across the entire population will then be 0. Now, suppose that we remove all individuals with a negative value of that trait (we could suppose that environmental circumstances changed such that positive values of the trait were extremely advantageous). The range of the trait is now x to 0, and its average value is x/2. Now, if we suppose that this trait arose by a process of variation that tends to extend the extremes of the range of values by an additional degree that we will call y. Even if no further selection acts upon this population, so long as y is large enough to counter the blending of a trait towards the average, the extreme values will persist; if y is greater than that, the range of variation will increase, and when y=x, we will have the full range of variation that existed in the original population, except in our selected sample with a mean at x/2 rather than at 0. Obviously, further selection can drive the average value ot this trait further in the same direction. Thus, Darwin's model of blending inheritance can be made to work.
Now, of course, this is all ultimately irrelevant: inheritance does not work that way. But I feel it important to spell out how it could have worked were it true, so as to illustrate how Darwin's theory was not significantly threatened by his defective model of genetics. I do not know whether Darwin himself thought things through this far, or whether any of his contemporaries did -- or indeed whether anyone has -- but I believe this to be a reasonable model.
That having been discussed, on with the chapter! Darwin starts out with observations on the effect of environment on the adult form, which he correctly infers to influence succeeding generations only when it affects the gametes (produced by what he calls the "generative organs" or gonads, and upon which he focuses his attention). It is in fact known that an individual's development can be altered, sometimes significantly, by the environmental conditions under which it was brought up: the resulting differences delineate what are called ecotypes. It is important to note that the capacity to produce all of the forms present in the population is still (potentially, at least) inherited, even if it is not displayed in the population. To give an example, a high-altitude ecotype of a plant might have leaves of a different size or shape than those of a low-altitude ecotype, but seeds from either ecotype will produce plants that match their environment rather than their parents in those features characteristic of the ecotype. Confusingly, the capacity to adapt in the course of development (necessary to produce different ecotypes) can itself be subject to genetic variation, such that one population might have a more extreme range of ecotypes than another. Darwin is aware of this distinction, although the terminology had not yet been invented, and correctly dismisses it in his discussion, since it only confuses matters. Evolution can only act upon heritable traits, and if a trait can be shown to be influenced by its environment, it becomes overly complicated for use as a model of evolution.
The next section of this chapter covers another aspect of biology that Darwin got wrong: he assumed that the effects of use or disuse were heritable. This is similar to Lamarck's concept of evolution, in which an individual improving some faculty or structure in the course of its life passes on such improvements to its offspring. (The famous example of this is the giraffe, which Lamarck supposed originally had a neck no longer than that of any other hoofed mammal, but which over successive generations kept straining to reach ever higher leaves, and thereby managed to lengthen its neck.) However, Darwin does not long dwell upon this, and does not suppose it to be as important an evolutionary force as natural selection. He supposes the reduction of eyes in subterranean animals, and the wings of island birds and beetles, to be due to the continued effects of disuse, and yet if one looks at the level of the population rather than of the individual, this is not an unreasonable shorthand. In discussing such examples, interestingly, this section deals less with the inheritance of acquired characteristics (to use the proper terminology for Lamarckian evolution) than it does with biogeography: the tendency of organisms in a given region to be related to one another, rather than to their counterparts from similar ecosystems in more distant regions.
The next section, on acclimatisation, touches on these ideas as well, but quickly returns to the concept of the ecotype. Darwin was astute to discern the difference between these phenomena, even as he was ignorant of the facts behind them. He even goes so far as to propose experiments to test the limits of acclimatisation, in order to suggest that the boundaries between ecotype and inherited variation were not yet then known.
Darwin next addresses "Correlation of Growth", which he ties quickly to Richard Owen's concept of serial homology, developed more or less to its present form about a decade previously. Perhaps the most significant aspect of this, understated by just about everybody, is the homology inherent in symmetry. In other words, the structures on the left side of the body are homologous to their counterparts on the right; and the developmental processes that influence one side will also influence the other. Similarly, processes that affect an animal's forelimbs will tend as well to affect its hind limbs. This is not absolute, of course; the dissimilarly sized claws of fiddler crabs are one clear example of symmetry being broken, and one of the most striking differences between humans and our closest relatives is the much greater length of our legs and of their arms. In any event, the importance of development to evolution is immense, and Darwin had clearly grasped this. He also pointed out that correlation between structures need not involve any developmental linkage: closely related species will tend to have traits in common due to simple inheritance, and developmental correlation need not be invoked.
Darwin proceeds from there to speculate upon an idea popular in his time, of a sort of balance across all parts of the developing body, such that overdevelopment in one part draws resources from elsewhere and results in underdevelopment in another part. He is circumspect about this, though, and unwilling to draw any lines between this effect and that of a combination of natural selection and inherited reduction of unusued body parts. He streamlines this further, with the adage that "natural selection is continually trying to economise in every part of the organisation." This is not a bad personification of the process of evolution, although now we would see developmental correlation and genetic drift as more significant players in the reduction of unused characters. As a result, less-derived organisms (or, as Darwin would put it, "those lower on the scale") show a greater degree of similarity in serially homologous parts: they have had less evolutionary pressure to specialise individual organs, which renders all of them more similar to one another. He concludes this section by pointing out that unused structures are prone to vary more. In other words, if there is no natural selection on a trait, it is free to vary. All that remains to bring this to the modern concept of genetic drift is the notion of fixation, that in a sufficiently small population some nonselected character states will be lost simply by chance.
This brings us to the halfway point in this chapter; I shall tackle the remaining half tomorrow.
To put it another way, suppose that a natural amount of variation exists in a population, such that some trait can be measured as being anywhere from x to -x, and all values have an even distribution. The average value of the trait across the entire population will then be 0. Now, suppose that we remove all individuals with a negative value of that trait (we could suppose that environmental circumstances changed such that positive values of the trait were extremely advantageous). The range of the trait is now x to 0, and its average value is x/2. Now, if we suppose that this trait arose by a process of variation that tends to extend the extremes of the range of values by an additional degree that we will call y. Even if no further selection acts upon this population, so long as y is large enough to counter the blending of a trait towards the average, the extreme values will persist; if y is greater than that, the range of variation will increase, and when y=x, we will have the full range of variation that existed in the original population, except in our selected sample with a mean at x/2 rather than at 0. Obviously, further selection can drive the average value ot this trait further in the same direction. Thus, Darwin's model of blending inheritance can be made to work.
Now, of course, this is all ultimately irrelevant: inheritance does not work that way. But I feel it important to spell out how it could have worked were it true, so as to illustrate how Darwin's theory was not significantly threatened by his defective model of genetics. I do not know whether Darwin himself thought things through this far, or whether any of his contemporaries did -- or indeed whether anyone has -- but I believe this to be a reasonable model.
That having been discussed, on with the chapter! Darwin starts out with observations on the effect of environment on the adult form, which he correctly infers to influence succeeding generations only when it affects the gametes (produced by what he calls the "generative organs" or gonads, and upon which he focuses his attention). It is in fact known that an individual's development can be altered, sometimes significantly, by the environmental conditions under which it was brought up: the resulting differences delineate what are called ecotypes. It is important to note that the capacity to produce all of the forms present in the population is still (potentially, at least) inherited, even if it is not displayed in the population. To give an example, a high-altitude ecotype of a plant might have leaves of a different size or shape than those of a low-altitude ecotype, but seeds from either ecotype will produce plants that match their environment rather than their parents in those features characteristic of the ecotype. Confusingly, the capacity to adapt in the course of development (necessary to produce different ecotypes) can itself be subject to genetic variation, such that one population might have a more extreme range of ecotypes than another. Darwin is aware of this distinction, although the terminology had not yet been invented, and correctly dismisses it in his discussion, since it only confuses matters. Evolution can only act upon heritable traits, and if a trait can be shown to be influenced by its environment, it becomes overly complicated for use as a model of evolution.
The next section of this chapter covers another aspect of biology that Darwin got wrong: he assumed that the effects of use or disuse were heritable. This is similar to Lamarck's concept of evolution, in which an individual improving some faculty or structure in the course of its life passes on such improvements to its offspring. (The famous example of this is the giraffe, which Lamarck supposed originally had a neck no longer than that of any other hoofed mammal, but which over successive generations kept straining to reach ever higher leaves, and thereby managed to lengthen its neck.) However, Darwin does not long dwell upon this, and does not suppose it to be as important an evolutionary force as natural selection. He supposes the reduction of eyes in subterranean animals, and the wings of island birds and beetles, to be due to the continued effects of disuse, and yet if one looks at the level of the population rather than of the individual, this is not an unreasonable shorthand. In discussing such examples, interestingly, this section deals less with the inheritance of acquired characteristics (to use the proper terminology for Lamarckian evolution) than it does with biogeography: the tendency of organisms in a given region to be related to one another, rather than to their counterparts from similar ecosystems in more distant regions.
The next section, on acclimatisation, touches on these ideas as well, but quickly returns to the concept of the ecotype. Darwin was astute to discern the difference between these phenomena, even as he was ignorant of the facts behind them. He even goes so far as to propose experiments to test the limits of acclimatisation, in order to suggest that the boundaries between ecotype and inherited variation were not yet then known.
Darwin next addresses "Correlation of Growth", which he ties quickly to Richard Owen's concept of serial homology, developed more or less to its present form about a decade previously. Perhaps the most significant aspect of this, understated by just about everybody, is the homology inherent in symmetry. In other words, the structures on the left side of the body are homologous to their counterparts on the right; and the developmental processes that influence one side will also influence the other. Similarly, processes that affect an animal's forelimbs will tend as well to affect its hind limbs. This is not absolute, of course; the dissimilarly sized claws of fiddler crabs are one clear example of symmetry being broken, and one of the most striking differences between humans and our closest relatives is the much greater length of our legs and of their arms. In any event, the importance of development to evolution is immense, and Darwin had clearly grasped this. He also pointed out that correlation between structures need not involve any developmental linkage: closely related species will tend to have traits in common due to simple inheritance, and developmental correlation need not be invoked.
Darwin proceeds from there to speculate upon an idea popular in his time, of a sort of balance across all parts of the developing body, such that overdevelopment in one part draws resources from elsewhere and results in underdevelopment in another part. He is circumspect about this, though, and unwilling to draw any lines between this effect and that of a combination of natural selection and inherited reduction of unusued body parts. He streamlines this further, with the adage that "natural selection is continually trying to economise in every part of the organisation." This is not a bad personification of the process of evolution, although now we would see developmental correlation and genetic drift as more significant players in the reduction of unused characters. As a result, less-derived organisms (or, as Darwin would put it, "those lower on the scale") show a greater degree of similarity in serially homologous parts: they have had less evolutionary pressure to specialise individual organs, which renders all of them more similar to one another. He concludes this section by pointing out that unused structures are prone to vary more. In other words, if there is no natural selection on a trait, it is free to vary. All that remains to bring this to the modern concept of genetic drift is the notion of fixation, that in a sufficiently small population some nonselected character states will be lost simply by chance.
This brings us to the halfway point in this chapter; I shall tackle the remaining half tomorrow.
Tuesday, 3 February 2009
The Origin, Chapter Four, Part Two
In the beginning of the section on extinction we find an amusingly extreme understatement: "Rarity, as geology tells us, is the precursor to extinction." But this is followed by a comment to the effect that the fluctuations in population size will make smaller populations are more likely to go extinct than larger ones: and the same could be said of allele frequencies in genes, which is a respelling of the concept of genetic drift. Darwin could have gone so much farther with that idea, but it is impressive that he got as far as he did. Darwin's point here appears primarily to be a reminder that extinction is a real phenomenon, and to reiterate and emphasise his earlier point, that competition between very similar species is the most severe. I am not sure whether this is in fact the case, but if so, it gives a nice mechanism for the quick and clean separation into different species of differently constituted but overlapping populations of the same species.
The next section, on "Divergence of Character", is an effort to explain how the slight differences between populations can become the stronger differences between species. He strikes analogy again with artificial selection: breeders, when selecting from amongst their charges, will choose those individuals most characteristic of the breed in question, which will tend to be the more extreme in those traits when compared to the wild stock. The point here is that selection acts continually over a long period in the same direction, and exaggerates those traits which it selects. Because of this, local populations become specialised. This results in another phenomenon that he notes next, that invasive species (to use the modern term) tend to come from different genera from those of the native populations: they have evolved from dissimilar species to fit similar niches. He draws an analogy between different species in an ecosystem to different systems in a single organism, thereby anticipating the Gaia hypothesis! On a more respectable front, however, looking at the analogy from the opposite direction offers a description of the evolution of multicellularity, in which early organisms were largely homogeneous, and easily outcompeted by those organisms whose cells started to specialise.
At this point Darwin introduces us to the only diagram in his book, which is also almost an early cladogram. Darwin takes some trouble to explain it; to those of us used to reading cladograms, it offers nothing new, but he offers more than cladograms do here as well. In this diagram, unlike in cladograms, the horizontal axis has meaning. It is well-designed for his purpose, as it simultaneously demonstrates diversification, extinction, and the varying genetic relationships between descendants that we now call phylogeny. He also touches on an important point made by Stephen Jay Gould in Wonderful Life, that the history of life is not one of increasing diversity from a small group of ancestors, but one of pruning of radically different branches of the tree and their subsequent replacement by radically different descendents of the few survivors. Darwin posits that life becomes more varied over time, but here he is not necessarily correct: in some ways (as Gould points out) the variety of life diminishes as time goes on and fundamentally different lineages are wiped out. While the surviving lineages may diversify, even into niches unoccupied by the previous assemblage, they are built on fewer platforms than at one time existed.
Darwin concludes the chapter with a summary, which offers nothing new, aside from an explicit description of a concept that is still current, that of the Tree of Life. While this has come under attack of late (endosymbiosis and gene transfer between unrelated groups providing two significant challenges), it is still a useful metaphor -- especially so in macroscopic organisms. Darwin also employs some very nice language. To give an example: "It is a truly wonderful fact -- the wonder of which we are apt to overlook from familiarity -- that all animals and all plants throughout all time and space should be related to each other in group subordinate to group…." And another: "As buds give rise by growth to fresh buds, and these, if vigorous, branch out and overtop on all sides many a feebler branch, so by generation I believe it has been with the great Tree of Life, which fills with its dead and broken branches the crust of the earth, and covers the surface with its ever branching and beautiful ramifications." Darwin is not done yet with the fundamentals of this theory, but he has still managed to encapsulate its essence admirably.
The next section, on "Divergence of Character", is an effort to explain how the slight differences between populations can become the stronger differences between species. He strikes analogy again with artificial selection: breeders, when selecting from amongst their charges, will choose those individuals most characteristic of the breed in question, which will tend to be the more extreme in those traits when compared to the wild stock. The point here is that selection acts continually over a long period in the same direction, and exaggerates those traits which it selects. Because of this, local populations become specialised. This results in another phenomenon that he notes next, that invasive species (to use the modern term) tend to come from different genera from those of the native populations: they have evolved from dissimilar species to fit similar niches. He draws an analogy between different species in an ecosystem to different systems in a single organism, thereby anticipating the Gaia hypothesis! On a more respectable front, however, looking at the analogy from the opposite direction offers a description of the evolution of multicellularity, in which early organisms were largely homogeneous, and easily outcompeted by those organisms whose cells started to specialise.
At this point Darwin introduces us to the only diagram in his book, which is also almost an early cladogram. Darwin takes some trouble to explain it; to those of us used to reading cladograms, it offers nothing new, but he offers more than cladograms do here as well. In this diagram, unlike in cladograms, the horizontal axis has meaning. It is well-designed for his purpose, as it simultaneously demonstrates diversification, extinction, and the varying genetic relationships between descendants that we now call phylogeny. He also touches on an important point made by Stephen Jay Gould in Wonderful Life, that the history of life is not one of increasing diversity from a small group of ancestors, but one of pruning of radically different branches of the tree and their subsequent replacement by radically different descendents of the few survivors. Darwin posits that life becomes more varied over time, but here he is not necessarily correct: in some ways (as Gould points out) the variety of life diminishes as time goes on and fundamentally different lineages are wiped out. While the surviving lineages may diversify, even into niches unoccupied by the previous assemblage, they are built on fewer platforms than at one time existed.
Darwin concludes the chapter with a summary, which offers nothing new, aside from an explicit description of a concept that is still current, that of the Tree of Life. While this has come under attack of late (endosymbiosis and gene transfer between unrelated groups providing two significant challenges), it is still a useful metaphor -- especially so in macroscopic organisms. Darwin also employs some very nice language. To give an example: "It is a truly wonderful fact -- the wonder of which we are apt to overlook from familiarity -- that all animals and all plants throughout all time and space should be related to each other in group subordinate to group…." And another: "As buds give rise by growth to fresh buds, and these, if vigorous, branch out and overtop on all sides many a feebler branch, so by generation I believe it has been with the great Tree of Life, which fills with its dead and broken branches the crust of the earth, and covers the surface with its ever branching and beautiful ramifications." Darwin is not done yet with the fundamentals of this theory, but he has still managed to encapsulate its essence admirably.
Monday, 2 February 2009
The Origin, Chapter Four, Part One
Darwin begins this chapter by stating the obvious conclusions from even the most casual synthesis of the previous three. The logic is simple:
Darwin was right to regard this as a significant force in evolution, but even here, immediately after his formal introduction of the topic, he hastens to add that it is not the only force: "Variations neither useful nor injurious would not be affected by natural selection, and would be left a fluctuating element". Were he to have elaborated on this more fully, he would have developed the theory of genetic drift; as it is, he has hinted at it strongly. He then moves onto another force in the evolution of populations, that of immigration. In other words, the influx or efflux of individuals can affect the course of a community's development, and can assist or hamper the force of selection on those that stay. As with drift, this is an important alternative to natural selection. Students of evolution who are counting along will note that the only major evolutionary force that has not been accounted for here (variation comprising at least in part the force of mutation) is sexual selection, which is another topic that Darwin explicitly developed. In other words, barely two paragraphs into the chapter, we have already been introduced to the gamut of evolutionary thinking, albeit in a preliminary and decidedly non-quantitative way. Darwin's view of evolution was impressively comprehensive. The whirlwind tour through modern evolutionary concepts is complemented by a modern ecological concept, that of invasive species, although not by that term. These, Darwin explains, alter the dynamic of their new environment and thus the nature of the features of its native inhabitants most useful to increasing their numbers.
Now he ties in the first chapter, comparing natural with artificial selection. There are of course differences: one is that natural selection has had a much greater length of time in which to work to produce extant organisms. But perhaps more importantly, "nature cares nothing for appearances, except in so far as they may be useful to any being. She can act on every internal organ, on every shade of constitutional difference, on the whole machinery of life." In other words, natural selection can influence changes too complex and subtle for human breeders to understand, and such changes are always useful to their possessors, rather than to the occasionally arbitrary preferences of human masters. "How fleeting are the wishes and efforts of man! how short his time!" -- and so what nature can do to life is immeasurably greater than what humanity has been able to accomplish (although of course genetic manipulation rather changes that picture!).
Darwin then goes on with more concepts that extend the theory of evolution into modern concepts. For one, he gives another passing mention of correlation of traits, which includes both what would become known as pleiotropy (wherein one gene controls more than one trait) and genetic linkage (in which multiple genes are inherited together). For another, he makes an even more passing comment in the direction of kin selection: "In social animals [natural selection] will adapt the structure of each individual for the benefit of the community; if each in consequence profits by the selected change." This smacks as well of a related but far more contentious topic, that of group selection; at least in this passage it is not clear how far and in which direction Darwin thought this sort of process could be taken. The principal point that Darwin is driving at here, though, is what he already touched on: natural selection acts for the benefit of each species in question, and for no-one else. Every adaptation brought about through natural selection is an adaptation to help the species possessing it in its environment, and no harmful change will persist. This is of course not necessarily the case, given the complexity of environment and inheritance: a classic case is that in which a mutation that provides resistance to malaria can also cause sickle-cell anemia, depending on how many copies of that gene an individual has. But even here, there is a trade-off that benefits the individual: so long as the probability that one will be exposed to malaria is higher than the probability that one will have the sickle-cell trait, the mutation in question will persist.
At this point, Darwin gives us a section heading, and introduces already his other major contribution to the theory of evolution: sexual selection. He would have much more to say on this in later work (particularly in The Descent of Man), but here brushes by the subject's two main factors: competition between members of the same sex for access to members of the opposite sex (exemplified by fights for dominance), and the tendency for members of one sex to prefer some and not other individuals of the opposite sex (exemplified by the bright plumage of many male birds). The subtleties between these two types of sexual selection are here unexplored, but they are at least mentioned.
Darwin proceeds with two illustrations of the principle of natural selection. The first focuses on wolves; several possibilities are presented in which a population might change over time, driven by environmental biasses. Importantly, the traits that Darwin supposes to be heritable and significant include behavioural ones. His second example is a compound one, showing how flowers and pollinating insects must co-evolve as each adapts to changes in the other. It is in this process that a species might exhibit traits that appear to benefit another species -- but if that other species provides some essential service to the first, then those traits that benefit the other are indirectly benefitting the first as well. He concludes this section with a reminder that natural selection acts on small differences over vast periods of time, much like Lyell's uniformitarian geology -- a significant comparison, one which continues to be made.
The next section is entitled "On the Advantage of Intercrossing", but it could as easily been called "On the Disadvantages of Hermaphroditism", which is at least to the modern reader a somewhat clearer description of its topic. Ignorant though Darwin was of even classical genetics, he was still quite cognisant of (and states explicitly) the fact that nothing can persist indefinitely without at least occasional genetic interchange with other members of the same species. One can almost feel the perplexed wonder in this section. The topic is brought up in prelude to the next section, "Circumstances Favourable to Natural Selection": here, he posits that self-fertilising hermaphrodites (and by extension asexual organisms) are more likely to adapt to smaller-scale differences in their environments. In other words, the genetic interchange in obligately sexual organisms (like birds) will tend to even out differences over a wide area, making local adaptation unlikely unless the circumstances are widespread across many locales. (Darwin alludes to an ill-defined lack of vigour and fertility of self-crossers when compared to outcrossers which is explained by, though does not imply, inbreeding depression; the vagueness borne of his lack of understanding of genetics is dismissable, though, in his recognition of the importance of genetic interchange to the spread of favourable adaptations. One factor missing in this analysis is the possibility of different adaptations recombining in new ways that render the offspring carrying those combinations even better adapted -- a puzzling omission.) The discussion of range size continues, to compare the benefits bestowed upon a species living for a long time in small and isolated region with those incurred by a species living in a large and varied region. Darwin argues that the latter will produce a species more capable of fending off intruders from other regions, and alludes to the plights of species endemic to isolated areas, including oceanic islands and the continent of Australia. Natural selection is supposed to lead to such generalised species by, among other things, the possibilities of range fractionation allowing for local adaptations to arise which are then spread across the larger range when the environmental factors enforcing the fractionation are lifted. In other words, Darwin is supposing the larger region to allow for a genetic reservoir of local adaptations, a library of useful traits that can be called upon to
- Individuals amongst a population differ from one another. Some differences are useful in one's daily life, and confer an advantage to their possessors. Importantly, some of these differences are heritable: having a feature means that one's offspring will also have that feature.
- More individuals are produced in each generation than represent the previous generation, meaning that (if the population is stable) more are produced than can survive.
- Putting these together, those individuals with differences that help them in their lives will have a better chance at surviving than those without. Those differences that are heritable will be more prevalent in the succeeding generations (assuming that they continue to confer an advantage over their alternatives), and so the average individual from one generation will differ from the average individual from the next.
Darwin was right to regard this as a significant force in evolution, but even here, immediately after his formal introduction of the topic, he hastens to add that it is not the only force: "Variations neither useful nor injurious would not be affected by natural selection, and would be left a fluctuating element". Were he to have elaborated on this more fully, he would have developed the theory of genetic drift; as it is, he has hinted at it strongly. He then moves onto another force in the evolution of populations, that of immigration. In other words, the influx or efflux of individuals can affect the course of a community's development, and can assist or hamper the force of selection on those that stay. As with drift, this is an important alternative to natural selection. Students of evolution who are counting along will note that the only major evolutionary force that has not been accounted for here (variation comprising at least in part the force of mutation) is sexual selection, which is another topic that Darwin explicitly developed. In other words, barely two paragraphs into the chapter, we have already been introduced to the gamut of evolutionary thinking, albeit in a preliminary and decidedly non-quantitative way. Darwin's view of evolution was impressively comprehensive. The whirlwind tour through modern evolutionary concepts is complemented by a modern ecological concept, that of invasive species, although not by that term. These, Darwin explains, alter the dynamic of their new environment and thus the nature of the features of its native inhabitants most useful to increasing their numbers.
Now he ties in the first chapter, comparing natural with artificial selection. There are of course differences: one is that natural selection has had a much greater length of time in which to work to produce extant organisms. But perhaps more importantly, "nature cares nothing for appearances, except in so far as they may be useful to any being. She can act on every internal organ, on every shade of constitutional difference, on the whole machinery of life." In other words, natural selection can influence changes too complex and subtle for human breeders to understand, and such changes are always useful to their possessors, rather than to the occasionally arbitrary preferences of human masters. "How fleeting are the wishes and efforts of man! how short his time!" -- and so what nature can do to life is immeasurably greater than what humanity has been able to accomplish (although of course genetic manipulation rather changes that picture!).
Darwin then goes on with more concepts that extend the theory of evolution into modern concepts. For one, he gives another passing mention of correlation of traits, which includes both what would become known as pleiotropy (wherein one gene controls more than one trait) and genetic linkage (in which multiple genes are inherited together). For another, he makes an even more passing comment in the direction of kin selection: "In social animals [natural selection] will adapt the structure of each individual for the benefit of the community; if each in consequence profits by the selected change." This smacks as well of a related but far more contentious topic, that of group selection; at least in this passage it is not clear how far and in which direction Darwin thought this sort of process could be taken. The principal point that Darwin is driving at here, though, is what he already touched on: natural selection acts for the benefit of each species in question, and for no-one else. Every adaptation brought about through natural selection is an adaptation to help the species possessing it in its environment, and no harmful change will persist. This is of course not necessarily the case, given the complexity of environment and inheritance: a classic case is that in which a mutation that provides resistance to malaria can also cause sickle-cell anemia, depending on how many copies of that gene an individual has. But even here, there is a trade-off that benefits the individual: so long as the probability that one will be exposed to malaria is higher than the probability that one will have the sickle-cell trait, the mutation in question will persist.
At this point, Darwin gives us a section heading, and introduces already his other major contribution to the theory of evolution: sexual selection. He would have much more to say on this in later work (particularly in The Descent of Man), but here brushes by the subject's two main factors: competition between members of the same sex for access to members of the opposite sex (exemplified by fights for dominance), and the tendency for members of one sex to prefer some and not other individuals of the opposite sex (exemplified by the bright plumage of many male birds). The subtleties between these two types of sexual selection are here unexplored, but they are at least mentioned.
Darwin proceeds with two illustrations of the principle of natural selection. The first focuses on wolves; several possibilities are presented in which a population might change over time, driven by environmental biasses. Importantly, the traits that Darwin supposes to be heritable and significant include behavioural ones. His second example is a compound one, showing how flowers and pollinating insects must co-evolve as each adapts to changes in the other. It is in this process that a species might exhibit traits that appear to benefit another species -- but if that other species provides some essential service to the first, then those traits that benefit the other are indirectly benefitting the first as well. He concludes this section with a reminder that natural selection acts on small differences over vast periods of time, much like Lyell's uniformitarian geology -- a significant comparison, one which continues to be made.
The next section is entitled "On the Advantage of Intercrossing", but it could as easily been called "On the Disadvantages of Hermaphroditism", which is at least to the modern reader a somewhat clearer description of its topic. Ignorant though Darwin was of even classical genetics, he was still quite cognisant of (and states explicitly) the fact that nothing can persist indefinitely without at least occasional genetic interchange with other members of the same species. One can almost feel the perplexed wonder in this section. The topic is brought up in prelude to the next section, "Circumstances Favourable to Natural Selection": here, he posits that self-fertilising hermaphrodites (and by extension asexual organisms) are more likely to adapt to smaller-scale differences in their environments. In other words, the genetic interchange in obligately sexual organisms (like birds) will tend to even out differences over a wide area, making local adaptation unlikely unless the circumstances are widespread across many locales. (Darwin alludes to an ill-defined lack of vigour and fertility of self-crossers when compared to outcrossers which is explained by, though does not imply, inbreeding depression; the vagueness borne of his lack of understanding of genetics is dismissable, though, in his recognition of the importance of genetic interchange to the spread of favourable adaptations. One factor missing in this analysis is the possibility of different adaptations recombining in new ways that render the offspring carrying those combinations even better adapted -- a puzzling omission.) The discussion of range size continues, to compare the benefits bestowed upon a species living for a long time in small and isolated region with those incurred by a species living in a large and varied region. Darwin argues that the latter will produce a species more capable of fending off intruders from other regions, and alludes to the plights of species endemic to isolated areas, including oceanic islands and the continent of Australia. Natural selection is supposed to lead to such generalised species by, among other things, the possibilities of range fractionation allowing for local adaptations to arise which are then spread across the larger range when the environmental factors enforcing the fractionation are lifted. In other words, Darwin is supposing the larger region to allow for a genetic reservoir of local adaptations, a library of useful traits that can be called upon to
Thursday, 29 January 2009
The Origin, Chapter Three
Chapter Three is, in many ways, the beginning of ecology. One of the ecologists in our reading group said that very little in modern community ecology was not laid out there. The concept of organisms interacting with both their environment, other species, and other members of the same species, and indeed the word "competition" in its ecological sense, are if not Darwin's pioneering work alone at least popularised by him (I do not know enough of the history of biology to say which). This chapter was inspired primarily by the work of the economist Thomas Malthus, who analysed the relative birth and death rates in human populations. Darwin's genius was to take this to a mechanical level, without the action of individual intentions. It was one of those ideas which seems obvious in retrospect but which was groundbreaking when first presented. As I read it I kept thinking that this chapter would be an ideal assignment to an introductory ecology class: it really is that comprehensive, that modern. The majority of the work that followed (at least in community ecology) has been merely refinement, primarily through the application of mathematics.
The chapter focuses on what Darwin called the "struggle for existence". The language was meant only partly to be taken literally: while animals must in fact frequently struggle to avoid being eaten, or to subdue rivals, the concept can also be applied to plants (as Darwin did). Those organisms that are able to leave offspring have succeeded in that struggle: they have had to survive long enough to reach reproductive maturity. The struggle goes further for those organisms that must mate in order to reproduce, and still further for those that must nurture their offspring -- but again, the struggle can be metaphorical. Plants which release more pollen, for instance, might win their struggle against others of the same species, in that they would leave more offspring. The struggle to produce viable offspring is implicit in the amount of energy that must be expended to build another organism: pregnant and nursing mothers, for instance, "eat for two", and this intensifies the daily efforts of finding food.
The struggle for existence has its place in the theory of evolution by making explicit the fact that more offspring are produced in each generation than can survive to replace their parents. In a stable population, this means that some offspring will die without having themselves reproduced: they will have lost in Darwin's struggle. Meanwhile, organisms are also individuals, and as such they vary, as was discussed in the previous chapter. Some individuals succeed in the struggle by virtue of inherited characteristics that give them an advantage over their less-endowed kin, and will pass those characteristics on to their own offspring. These advantages may help adapt the organism to its physical environment, or they may help the organism find food or avoid becoming food, or they may help the organism increase its likelihood of reproducing, or the number or health of its offspring.
This leads to the concept of natural selection, which is the topic of the next chapter, but not without offering what must be one of the most obvious attempts at softening the blow in scientific history. Darwin's depiction of Nature is as a brutal and ultimately losing struggle to stay alive, wherein the best that any living thing can hope for is to leave successful offspring before it succumbs to the myriad and overwhelming forces arrayed against it. This must have been appalling to Victorian sensibilities, which Darwin attempts to soothe: "When we reflect on this struggle, we may console ourselves with the full belief, that the war of nature is not incessant, that no fear is felt, that death is generally prompt, and that the vigorous, the healthy, and the happy survive and multiply." I will not deny that there is much joy in life, but fear, sorrow, anger, and pain are also unavoidable. Darwin's empathy for living things is here curiously selective: what of the weak, the sickly, the despondent? Do they really feel no fear? Do they never suffer for long? Of course, the question must here be addressed as to whether other forms of life are capable of feeling fear or suffering pain. It is not my intention to debate that here, except to say that it was this same work of Darwin's that made this question scientifically viable.
But while such a passage fails in any way to support its implication that (in this context) ecological and (ultimately) evolutionary success leads to a more enjoyable life, its greatest failing lies in its assertion that the "struggle for existence" is not constant. Perhaps it is not constant in every aspect of life, but the interactions between living things are so complex that it cannot be assumed that, at one level or another, one is never at loggerheads with something. In finding shelter, we must displace others looking for that same resource. The very act of eating implies that something else had to die: how is that not part of the struggle? Our immune systems are continually fighting off infections -- and doing so the more militantly when we are not aware of it! In winning the hand of one's mate, another must lose the same; what is (one hopes) a happy union for some is necessarily a source of frustration and feelings of loss for others. Nature may not always be red in tooth and claw, but our interactions with one another and with other living things are rarely benign for all.
The chapter focuses on what Darwin called the "struggle for existence". The language was meant only partly to be taken literally: while animals must in fact frequently struggle to avoid being eaten, or to subdue rivals, the concept can also be applied to plants (as Darwin did). Those organisms that are able to leave offspring have succeeded in that struggle: they have had to survive long enough to reach reproductive maturity. The struggle goes further for those organisms that must mate in order to reproduce, and still further for those that must nurture their offspring -- but again, the struggle can be metaphorical. Plants which release more pollen, for instance, might win their struggle against others of the same species, in that they would leave more offspring. The struggle to produce viable offspring is implicit in the amount of energy that must be expended to build another organism: pregnant and nursing mothers, for instance, "eat for two", and this intensifies the daily efforts of finding food.
The struggle for existence has its place in the theory of evolution by making explicit the fact that more offspring are produced in each generation than can survive to replace their parents. In a stable population, this means that some offspring will die without having themselves reproduced: they will have lost in Darwin's struggle. Meanwhile, organisms are also individuals, and as such they vary, as was discussed in the previous chapter. Some individuals succeed in the struggle by virtue of inherited characteristics that give them an advantage over their less-endowed kin, and will pass those characteristics on to their own offspring. These advantages may help adapt the organism to its physical environment, or they may help the organism find food or avoid becoming food, or they may help the organism increase its likelihood of reproducing, or the number or health of its offspring.
This leads to the concept of natural selection, which is the topic of the next chapter, but not without offering what must be one of the most obvious attempts at softening the blow in scientific history. Darwin's depiction of Nature is as a brutal and ultimately losing struggle to stay alive, wherein the best that any living thing can hope for is to leave successful offspring before it succumbs to the myriad and overwhelming forces arrayed against it. This must have been appalling to Victorian sensibilities, which Darwin attempts to soothe: "When we reflect on this struggle, we may console ourselves with the full belief, that the war of nature is not incessant, that no fear is felt, that death is generally prompt, and that the vigorous, the healthy, and the happy survive and multiply." I will not deny that there is much joy in life, but fear, sorrow, anger, and pain are also unavoidable. Darwin's empathy for living things is here curiously selective: what of the weak, the sickly, the despondent? Do they really feel no fear? Do they never suffer for long? Of course, the question must here be addressed as to whether other forms of life are capable of feeling fear or suffering pain. It is not my intention to debate that here, except to say that it was this same work of Darwin's that made this question scientifically viable.
But while such a passage fails in any way to support its implication that (in this context) ecological and (ultimately) evolutionary success leads to a more enjoyable life, its greatest failing lies in its assertion that the "struggle for existence" is not constant. Perhaps it is not constant in every aspect of life, but the interactions between living things are so complex that it cannot be assumed that, at one level or another, one is never at loggerheads with something. In finding shelter, we must displace others looking for that same resource. The very act of eating implies that something else had to die: how is that not part of the struggle? Our immune systems are continually fighting off infections -- and doing so the more militantly when we are not aware of it! In winning the hand of one's mate, another must lose the same; what is (one hopes) a happy union for some is necessarily a source of frustration and feelings of loss for others. Nature may not always be red in tooth and claw, but our interactions with one another and with other living things are rarely benign for all.
Sunday, 25 January 2009
The Origin, Chapter Two
Chapters Two and Three will be discussed this week together. They lay the foundation for the principle of Natural Selection, covering respectively variation in nature and the "struggle for existence", the principle wherein with each generation many more offspring are generated than can survive.
Chapter Two is only twenty pages long in the online facsimile, and alludes to Darwin's never-written follow-up to the Origin, in which he planned on expanding on the Origin to his satisfaction. As it happened, the degree of detail provided in this work proved sufficient, and the "long catalogue of dry facts" that would have constituted much of the successor unnecessary.
The question of variation within species brings up the question of what constitutes a species, which is a contentious issue today. Back in Darwin's day, the question was just as open, although the candidate answers were very different. One of the more popular notions was that each species was created independently, and that "varieties" (sub-species and so on, a term used here with some scientific force, and somewhat interchangeable with "race") had obviously descended from and were members of the same species. At the same time, then as now, "every naturalist knows vaguely what he means when he speaks of a species." It is odd how some things never change!
Again anticipating the study of ecology, Darwin notes that features found to be useful in one individual of a species may not be so useful in another. He alludes to thickness of coat as an example: this is a more important feature to an animal in an extreme environment than to one in a more temperate locale, although the capacity to vary its thickness seasonally may be more important to the resident of the temperate zone. What varies in a species has traditionally not been considered important, but Darwin notes the circularity of the argument in which "important organs never vary", thereby removing such variation from consideration by using it to divide organisms into different species. Yet, substantial variation in unambiguously important organs can be found within otherwise-defined species: what we now know to be developmental variation in insects is noted by Darwin to produce very different morphologies in the paths taken by individual nerves.
The problem is most severe in what Darwin calls "protean" or "polymorphic" species. Such species (or indeed genera), claims Darwin, tend to be highly variable everywhere that they have been found, and in the case of fossils, in every period. This suggests that plasticity in development is itself an inheritable trait, at least inasmuch as that the corrective forces keeping development on a particular path are stronger in less-variable species, and weaker in more-variable ones. Darwin notes that the parts of organisms that vary are likely of less value to their survival than are the parts that do not, which is a necessary observation of any system in which natural selection is in operation.
Darwin next delves into the question of terminology, addressing the question of how much variation can be tolerated within a given species. Essentially, he says, the question cannot easily be settled; every specialist has a different list of valid species. He brushes by the distinction between allopatric and sympatric species, which would become important to evolutionary theorists in years to come. Here he merely notes that geography can be as characteristic as is form to determining species boundaries. Ultimately, the question of dividing individual species must be done using objective criteria, something that still has not been settled today; to do otherwise is "vainly to beat the air." Darwin briefly mentions what are now called "ring species", in which neighbouring forms are obviously continuous species, but members of the extremes of the range are distinct and do not easily interbreed.
The degree of known variation within a group of organisms is in no small part a function of the degree to which it has been studied. This is, of course, obvious, but it has important implications to the distinction of individual species, and Darwin goes from there to point out that it is much easier to learn a new group of organisms by taking their representatives from a particular locale in isolation; to consider initially those from other locations as well tends to confuse things. Once familiarity with a particular flora or fauna is attained, though, extending one's range of observations will challange one's sense of "species", both usefully and otherwise, as what could be diagnostic for a species in one region may ultimately prove highly variable between regions.
Having shown how variable individual species may be, and having suggested almost in passing that varieties within a species may in fact be incipient species, Darwin makes the important point that such varieties need not ultimately become different species. He goes on to point out that larger genera (those with more species) are both more widely distributed and more dominant than smaller genera, a point which I am not sure holds today. There are, at the very least, certainly exceptions, and Darwin points to a few of them himself. In particular, those species adapted to unusual environments (aquatic plants, for instance), tend to be more uniform. The other case that he gives, "plants low in the scale of organisation" being more widely distributed than their "higher" counterparts, I find definitely doubtful, on a similar principle to Darwin's own above. While individual knowledge of variation increases (at the very least) one's capacity to erect species distinctions, the features which vary in "lowly" organisms do so less markedly, and so the important distinctions are all the more subtle. To make a modern point of this, while they may not have evolved as far from the common ancestor as "higher" forms, they have nevertheless been evolving for the same length of time, and often for the same number of generations, and have had every opportunity to vary in ways that we may not appreciate.
An unwritten generalisation follows: Darwin believes nature to be essentially uniformitarian. This was a controversial geological principle when Darwin left on the Beagle, stating that the processes which shaped the Earth in the past are still in effect today. There are too many known exceptions for this to be considered strictly true today, and many of the processes which we now understand to be in effect (plate tectonics, for instance) were unsuspected in the early nineteenth century, but the essential notion is correct. Darwin's application of this to biology is a cornerstone to the science. In other words, evolution has happened, and it is continuing to happen today: it can be made an experimental science. These notions are not explicit in the Origin (at least not at this point in the work), but they are very much implied.
In summary, this chapter appears to make two important main points. First, the larger genera (by which is meant the more speciose genera), the more varieties can be found within each of its species. A related though distinct point is that the threshold of variation characterising each of those species and varieties is lower than that for the smaller genera, within which species and varieties tend to be more distinct from one another. As I said above, I am not sure that this is actually the case, and in any event, I do not see how it is critical to Darwin's argument. Far more important is this chapter's other point, that variation is a continuum: the more closely one looks, the more similarities one finds between different groups, and the more individual differences one sees within individual groups. Species are artificial constructs, conveniences for human classification, and are not always unambiguously delineated. Furthermore, "little groups of species are generally clustered like satellites around certain other species", by which Darwin means that genera may be divided into subgenera, the species of which resemble one another to a greater degree than they do members of other subgenera, and which tend to be found in closer proximity to one another geographically as well as morphologically. This is not an unimportant point: the continuity between species is not evenly distributed, and the fuzziness of the species definition can be extended to higher levels as well.
Chapter Two is only twenty pages long in the online facsimile, and alludes to Darwin's never-written follow-up to the Origin, in which he planned on expanding on the Origin to his satisfaction. As it happened, the degree of detail provided in this work proved sufficient, and the "long catalogue of dry facts" that would have constituted much of the successor unnecessary.
The question of variation within species brings up the question of what constitutes a species, which is a contentious issue today. Back in Darwin's day, the question was just as open, although the candidate answers were very different. One of the more popular notions was that each species was created independently, and that "varieties" (sub-species and so on, a term used here with some scientific force, and somewhat interchangeable with "race") had obviously descended from and were members of the same species. At the same time, then as now, "every naturalist knows vaguely what he means when he speaks of a species." It is odd how some things never change!
Again anticipating the study of ecology, Darwin notes that features found to be useful in one individual of a species may not be so useful in another. He alludes to thickness of coat as an example: this is a more important feature to an animal in an extreme environment than to one in a more temperate locale, although the capacity to vary its thickness seasonally may be more important to the resident of the temperate zone. What varies in a species has traditionally not been considered important, but Darwin notes the circularity of the argument in which "important organs never vary", thereby removing such variation from consideration by using it to divide organisms into different species. Yet, substantial variation in unambiguously important organs can be found within otherwise-defined species: what we now know to be developmental variation in insects is noted by Darwin to produce very different morphologies in the paths taken by individual nerves.
The problem is most severe in what Darwin calls "protean" or "polymorphic" species. Such species (or indeed genera), claims Darwin, tend to be highly variable everywhere that they have been found, and in the case of fossils, in every period. This suggests that plasticity in development is itself an inheritable trait, at least inasmuch as that the corrective forces keeping development on a particular path are stronger in less-variable species, and weaker in more-variable ones. Darwin notes that the parts of organisms that vary are likely of less value to their survival than are the parts that do not, which is a necessary observation of any system in which natural selection is in operation.
Darwin next delves into the question of terminology, addressing the question of how much variation can be tolerated within a given species. Essentially, he says, the question cannot easily be settled; every specialist has a different list of valid species. He brushes by the distinction between allopatric and sympatric species, which would become important to evolutionary theorists in years to come. Here he merely notes that geography can be as characteristic as is form to determining species boundaries. Ultimately, the question of dividing individual species must be done using objective criteria, something that still has not been settled today; to do otherwise is "vainly to beat the air." Darwin briefly mentions what are now called "ring species", in which neighbouring forms are obviously continuous species, but members of the extremes of the range are distinct and do not easily interbreed.
The degree of known variation within a group of organisms is in no small part a function of the degree to which it has been studied. This is, of course, obvious, but it has important implications to the distinction of individual species, and Darwin goes from there to point out that it is much easier to learn a new group of organisms by taking their representatives from a particular locale in isolation; to consider initially those from other locations as well tends to confuse things. Once familiarity with a particular flora or fauna is attained, though, extending one's range of observations will challange one's sense of "species", both usefully and otherwise, as what could be diagnostic for a species in one region may ultimately prove highly variable between regions.
Having shown how variable individual species may be, and having suggested almost in passing that varieties within a species may in fact be incipient species, Darwin makes the important point that such varieties need not ultimately become different species. He goes on to point out that larger genera (those with more species) are both more widely distributed and more dominant than smaller genera, a point which I am not sure holds today. There are, at the very least, certainly exceptions, and Darwin points to a few of them himself. In particular, those species adapted to unusual environments (aquatic plants, for instance), tend to be more uniform. The other case that he gives, "plants low in the scale of organisation" being more widely distributed than their "higher" counterparts, I find definitely doubtful, on a similar principle to Darwin's own above. While individual knowledge of variation increases (at the very least) one's capacity to erect species distinctions, the features which vary in "lowly" organisms do so less markedly, and so the important distinctions are all the more subtle. To make a modern point of this, while they may not have evolved as far from the common ancestor as "higher" forms, they have nevertheless been evolving for the same length of time, and often for the same number of generations, and have had every opportunity to vary in ways that we may not appreciate.
An unwritten generalisation follows: Darwin believes nature to be essentially uniformitarian. This was a controversial geological principle when Darwin left on the Beagle, stating that the processes which shaped the Earth in the past are still in effect today. There are too many known exceptions for this to be considered strictly true today, and many of the processes which we now understand to be in effect (plate tectonics, for instance) were unsuspected in the early nineteenth century, but the essential notion is correct. Darwin's application of this to biology is a cornerstone to the science. In other words, evolution has happened, and it is continuing to happen today: it can be made an experimental science. These notions are not explicit in the Origin (at least not at this point in the work), but they are very much implied.
In summary, this chapter appears to make two important main points. First, the larger genera (by which is meant the more speciose genera), the more varieties can be found within each of its species. A related though distinct point is that the threshold of variation characterising each of those species and varieties is lower than that for the smaller genera, within which species and varieties tend to be more distinct from one another. As I said above, I am not sure that this is actually the case, and in any event, I do not see how it is critical to Darwin's argument. Far more important is this chapter's other point, that variation is a continuum: the more closely one looks, the more similarities one finds between different groups, and the more individual differences one sees within individual groups. Species are artificial constructs, conveniences for human classification, and are not always unambiguously delineated. Furthermore, "little groups of species are generally clustered like satellites around certain other species", by which Darwin means that genera may be divided into subgenera, the species of which resemble one another to a greater degree than they do members of other subgenera, and which tend to be found in closer proximity to one another geographically as well as morphologically. This is not an unimportant point: the continuity between species is not evenly distributed, and the fuzziness of the species definition can be extended to higher levels as well.
Tuesday, 20 January 2009
The Origin, Chapter One
A couple of weeks ago, a friend asked whether there was a group in place to read Darwin's Origin of Species this year, on the 150th anniversary of its initial publication. I replied that I was not aware of any such group, and somehow wound up helping to found one. I had read the sixth and final edition before, and have decided to read the first edition for this group. (All editions are available for free online at Darwin Online.) We are having our initial meeting tomorrow, but I have had enough thoughts about the work that I felt it appropriate to record them here, before being shaped too much by others' interpretations.
The Introduction begins abruptly, so much so that I almost wondered whether there was a page missing in the submitted manuscript. It is written in a comparatively casual tone, almost as a letter to the reader, and nothing like one would expect of one of the most significant works of science ever published. It starts with a summary of the genesis of the work, in what would be a brief account in modern terms, and nothing short of astonishingly so in Darwin's Victorian terms. I should mention that, though I am tempted to cover this history here, it is not my point to summarise the book, so much as to critique it, from both a modern and (so far as I can succeed in placing myself into it) a contemporary context. My prior experience with it was positive enough that I would encourage others to read it for themselves; I flatter myself to think that this account might help in understanding Darwin's work, but I have no intention of replacing it. At the same time, Darwin regarded this 450-page book as an abstract, which only a Victorian could do. He apologises for not citing the work of others (although he does refer to such work frequently, and attributes it accordingly, he does not indicate where one might find it published), appealing to a lack of space -- again, a charmingly (some might say irritatingly) Victorian conceit in so large a work. I think it more reasonable to consider that Darwin was under considerable pressure to get his work published, since he had almost been scooped by Wallace. He gives an acknowledgement to his peers and mentors that (again) would be concise in modern terms, mentioning nobody by name aside from Hooker.
Then the Introduction begins as one might expect it to, setting the stage for the work to follow. Darwin first addresses the complexity not only of living things in and of themselves but as well that of their interactions with their environment and with other living things, which he supposes makes the proposition all the more marvellous that they arrived at their present state exclusively through natural processes. All this, he says, will be explained; and he feels it best to start with a discussion of a process analogous to his favoured mechanism, and so begins an outline of the book itself. The first part of the book (comprising five chapters) takes off from this point to discuss artificial selection, and moves on from there to cover variation in nature; the overproduction and differential survival of members of each successive variation; and the central principle of natural selection. He rounds this part of the book off with a discussion of the laws of variation and of correlation (of which I have more to say shortly). The second part of the book (the next four chapters) discusses possible objections to his theory: the complexity of any living thing and its constituent parts; the matter of instinct and other behavioural traits; the erection of biologically relevant boundaries between species; and the paucity of data from the fossil record. The third part of the book (another four chapters) picks up on this last theme, but move from addressing possible flaws in the theory to demonstrating its strengths: the continuity of living things from one form to another through time; two chapters on biogeography; and finally one on classification and comparative biology. Finally, he summarises the whole work in the last chapter.
Darwin finishes his Introduction with more uncharacteristic brevity, remarking with model honesty that the theory is far from finished, and while it is capable of explaining much, it does not cover everything. His last sentence is worthy of repetition: "Furthermore, I am convinced that Natural Selection has been the main but not exclusive means of modification." This is remarkable especially in that Darwin is often assumed to have been a strict adaptationist -- to have held that everything in life can be explained solely through natural selection acting on existing variation. This misconception has gone to the point of modern scientists claiming exactly that being called "Darwinians". Darwin was ahead of his time in many ways, and one of them was this very conviction.
Chapter One of the book (as mentioned) covers artificial selection. I have comparatively less to say on this; it is at this point that the Victorian locks are opened and the flood of words rushes through. Stylistic considerations aside, the fact that the meat of the book begins here means that the pace must necessarily slow somewhat. My earlier impression is not contradicted, though: Darwin remains quite readable, for all his verbosity.
The first chapter is divided into several sections in the table of contents, but these are lumped together by headings in the text itself, and by the running titles at the top of the page. The first such section deals with what Darwin knew of genetics, which is almost embarrassingly incorrect. It is truly impressive how much he got wrong -- not just wrong, but at times fantastically so -- and yet he managed to draw the right conclusions from such faulty premises. Much has been said of the irony of Darwin and Mendel being contemporaries unaware of one another's work, and of how the combination of their work led to a paradigm shift in biology almost as significant as that generated by each body of work taken by itself. Darwin assumed that inheritance was an analogue matter, that traits from each parent blended together rather than existed as isolatable units, and had no explanation for such phenomena as recessive traits or chromosomal linkage. At the same time, he was quite aware that some traits did seem to be connected, and suggests something suspiciously close to pleiotropy, in which a single gene affects multiple traits -- a situation which can explain the evolution of many seemingly neutral or even deleterious traits, assuming them to be so linked to beneficial traits. Darwin is similarly aware of reversions as significant, but has no appreciation for any mechanisms behind such phenomena. Then again, he is not concerned with the mechanisms of genetics, which is perhaps why is able to be so right in his conclusions while being so wrong in his premises. Another angle that Darwin plays a fair bit on is the origin of domestic animals and plants, on which he speculates somewhat unrestrainedly, but rarely far from what has become accepted today.
The second section of this chapter deals with one particular example of domestication. Having spent five years at sea, Darwin proceeded to invest himself in studies of individual groups of living things -- beetles, barnacles, earthworms, and pigeons. The latter provide the subject of this section. Here he is on much firmer footing. Darwin impresses, perhaps ostentatiously so, with his encyclopaedic knowledge of breeds and characteristics of pigeons, and gives examples of how some of the more extravagant breeds could have come about. This blends into the third section, which expands the discussion to cover other species, and from there to the fourth, which covers the mechanisms of selection -- but from an ecological rather than a genetic point of view. The fifth and sixth sections take this further, positing that much artificial selection is unconscious. There are thus two types of artificial selection at work, one planned and rapid, the other a slow and organic extension of the collective preference of those charged with keeping living things. Throughout these sections, this is where Darwin truly shines (I have read -- though I forget where -- of Darwin being regarded as a pioneer in ecology as well as evolution); he looks best at the big picture, even while supplying a torrent of fine details. At the same time, his arguments here are obvious, which is perhaps why his work made such an impression at the time. It is impossible not to agree with him.
A seventh section concerns the "circumstances of man's selection", in which Darwin refutes the notion that many domesticated varieties arose recently because their variability manifested itself only recently. Not so, claims Darwin: rather, it is only recently that human meticulousness and record-keeping rose to the level of being able to pick out such subtle differences as were always present but insufficiently selected before. At the same time, he falters a little here, in his assessment of the degrees of variability amongst different domesticated plants and animals. It is true that (as he suggests) individual species' behavioural attributes contribute (and this is an important factor which I am not sure others had noted beforehand), but there are many more dimensions to consider. Among other things, some species simply do have a greater tendency toward morphological variation than others. Time is a factor as well: something that has been bred for thousands of years will have had the chance to diverge much further from its original form than something that has only been bred for hundreds of years. Dogs and cats illustrate both of these points: dogs are not merely more tractable animals than cats -- they are in fact more plastic as a species, and they have been domesticated for far longer than cats (or, for that matter, anything else), giving far more time in which to exaggerate that plasticity.
Finally, Darwin gives us a single-page summary of the chapter, and a pithy ending: "Over all these causes of Change I am convinced that the accumulative action of Selection, whether applied methodically and more quickly, or unconsciously and more slowly, but more efficiently, is by far the predominant Power." Thus begins the book that changed biology.
The Introduction begins abruptly, so much so that I almost wondered whether there was a page missing in the submitted manuscript. It is written in a comparatively casual tone, almost as a letter to the reader, and nothing like one would expect of one of the most significant works of science ever published. It starts with a summary of the genesis of the work, in what would be a brief account in modern terms, and nothing short of astonishingly so in Darwin's Victorian terms. I should mention that, though I am tempted to cover this history here, it is not my point to summarise the book, so much as to critique it, from both a modern and (so far as I can succeed in placing myself into it) a contemporary context. My prior experience with it was positive enough that I would encourage others to read it for themselves; I flatter myself to think that this account might help in understanding Darwin's work, but I have no intention of replacing it. At the same time, Darwin regarded this 450-page book as an abstract, which only a Victorian could do. He apologises for not citing the work of others (although he does refer to such work frequently, and attributes it accordingly, he does not indicate where one might find it published), appealing to a lack of space -- again, a charmingly (some might say irritatingly) Victorian conceit in so large a work. I think it more reasonable to consider that Darwin was under considerable pressure to get his work published, since he had almost been scooped by Wallace. He gives an acknowledgement to his peers and mentors that (again) would be concise in modern terms, mentioning nobody by name aside from Hooker.
Then the Introduction begins as one might expect it to, setting the stage for the work to follow. Darwin first addresses the complexity not only of living things in and of themselves but as well that of their interactions with their environment and with other living things, which he supposes makes the proposition all the more marvellous that they arrived at their present state exclusively through natural processes. All this, he says, will be explained; and he feels it best to start with a discussion of a process analogous to his favoured mechanism, and so begins an outline of the book itself. The first part of the book (comprising five chapters) takes off from this point to discuss artificial selection, and moves on from there to cover variation in nature; the overproduction and differential survival of members of each successive variation; and the central principle of natural selection. He rounds this part of the book off with a discussion of the laws of variation and of correlation (of which I have more to say shortly). The second part of the book (the next four chapters) discusses possible objections to his theory: the complexity of any living thing and its constituent parts; the matter of instinct and other behavioural traits; the erection of biologically relevant boundaries between species; and the paucity of data from the fossil record. The third part of the book (another four chapters) picks up on this last theme, but move from addressing possible flaws in the theory to demonstrating its strengths: the continuity of living things from one form to another through time; two chapters on biogeography; and finally one on classification and comparative biology. Finally, he summarises the whole work in the last chapter.
Darwin finishes his Introduction with more uncharacteristic brevity, remarking with model honesty that the theory is far from finished, and while it is capable of explaining much, it does not cover everything. His last sentence is worthy of repetition: "Furthermore, I am convinced that Natural Selection has been the main but not exclusive means of modification." This is remarkable especially in that Darwin is often assumed to have been a strict adaptationist -- to have held that everything in life can be explained solely through natural selection acting on existing variation. This misconception has gone to the point of modern scientists claiming exactly that being called "Darwinians". Darwin was ahead of his time in many ways, and one of them was this very conviction.
Chapter One of the book (as mentioned) covers artificial selection. I have comparatively less to say on this; it is at this point that the Victorian locks are opened and the flood of words rushes through. Stylistic considerations aside, the fact that the meat of the book begins here means that the pace must necessarily slow somewhat. My earlier impression is not contradicted, though: Darwin remains quite readable, for all his verbosity.
The first chapter is divided into several sections in the table of contents, but these are lumped together by headings in the text itself, and by the running titles at the top of the page. The first such section deals with what Darwin knew of genetics, which is almost embarrassingly incorrect. It is truly impressive how much he got wrong -- not just wrong, but at times fantastically so -- and yet he managed to draw the right conclusions from such faulty premises. Much has been said of the irony of Darwin and Mendel being contemporaries unaware of one another's work, and of how the combination of their work led to a paradigm shift in biology almost as significant as that generated by each body of work taken by itself. Darwin assumed that inheritance was an analogue matter, that traits from each parent blended together rather than existed as isolatable units, and had no explanation for such phenomena as recessive traits or chromosomal linkage. At the same time, he was quite aware that some traits did seem to be connected, and suggests something suspiciously close to pleiotropy, in which a single gene affects multiple traits -- a situation which can explain the evolution of many seemingly neutral or even deleterious traits, assuming them to be so linked to beneficial traits. Darwin is similarly aware of reversions as significant, but has no appreciation for any mechanisms behind such phenomena. Then again, he is not concerned with the mechanisms of genetics, which is perhaps why is able to be so right in his conclusions while being so wrong in his premises. Another angle that Darwin plays a fair bit on is the origin of domestic animals and plants, on which he speculates somewhat unrestrainedly, but rarely far from what has become accepted today.
The second section of this chapter deals with one particular example of domestication. Having spent five years at sea, Darwin proceeded to invest himself in studies of individual groups of living things -- beetles, barnacles, earthworms, and pigeons. The latter provide the subject of this section. Here he is on much firmer footing. Darwin impresses, perhaps ostentatiously so, with his encyclopaedic knowledge of breeds and characteristics of pigeons, and gives examples of how some of the more extravagant breeds could have come about. This blends into the third section, which expands the discussion to cover other species, and from there to the fourth, which covers the mechanisms of selection -- but from an ecological rather than a genetic point of view. The fifth and sixth sections take this further, positing that much artificial selection is unconscious. There are thus two types of artificial selection at work, one planned and rapid, the other a slow and organic extension of the collective preference of those charged with keeping living things. Throughout these sections, this is where Darwin truly shines (I have read -- though I forget where -- of Darwin being regarded as a pioneer in ecology as well as evolution); he looks best at the big picture, even while supplying a torrent of fine details. At the same time, his arguments here are obvious, which is perhaps why his work made such an impression at the time. It is impossible not to agree with him.
A seventh section concerns the "circumstances of man's selection", in which Darwin refutes the notion that many domesticated varieties arose recently because their variability manifested itself only recently. Not so, claims Darwin: rather, it is only recently that human meticulousness and record-keeping rose to the level of being able to pick out such subtle differences as were always present but insufficiently selected before. At the same time, he falters a little here, in his assessment of the degrees of variability amongst different domesticated plants and animals. It is true that (as he suggests) individual species' behavioural attributes contribute (and this is an important factor which I am not sure others had noted beforehand), but there are many more dimensions to consider. Among other things, some species simply do have a greater tendency toward morphological variation than others. Time is a factor as well: something that has been bred for thousands of years will have had the chance to diverge much further from its original form than something that has only been bred for hundreds of years. Dogs and cats illustrate both of these points: dogs are not merely more tractable animals than cats -- they are in fact more plastic as a species, and they have been domesticated for far longer than cats (or, for that matter, anything else), giving far more time in which to exaggerate that plasticity.
Finally, Darwin gives us a single-page summary of the chapter, and a pithy ending: "Over all these causes of Change I am convinced that the accumulative action of Selection, whether applied methodically and more quickly, or unconsciously and more slowly, but more efficiently, is by far the predominant Power." Thus begins the book that changed biology.
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