- 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