Once again, this is just a repost of what I wrote in my book club discussion forum.
Chapter 2 – Variation Under Nature
This is a fairly short chapter, with much less information than the discussion of variation under domestication. In part, this is because a lot more was known about variation in domesticated animals and plants than in natural species. However, it wasn’t considered by everyone to be a good approach — Wallace always thought Darwin’s argument was weakened by relying so much on domestication as an analogy with natural processes.
Some things I found interesting in this chapter:
1) Species are hard to define. We talked in class about how there is no clear definition of species and how this causes problems in biology. Well, Darwin recognized the difficulty very early (p.67).
“Nor shall I here discuss the various definitions which have been given of the term species. No one definition has as yet satisfied all naturalists; yet every naturalist knows vaguely what he means when he speaks of a species.”
2) Darwin begins to suggest that the variation within species (e.g., varieties) is the same stuff that turns into differences among species. He introduces the term “incipient species” to indicate this. However, not all incipient species will become species — some may go exinct and some may not change further (p.76).
“Hence I believe a well-marked variety may be justly called an incipient species; but whether this belief be justifiable must be judged of by the general weight of the several facts and views given throughout this work.
It need not be supposed that all varieties or incipient species necessarily attain the rank of species. They may whilst in this incipient state become extinct, or they may endure as varieties for very long periods.”
3) Sometimes daughter species can co-exist with parental species — it is not always a gradual change of one species into another. In other words, Darwin recognizes cladogenesis (indeed, something compatible with punctuated equilibria) and not only anagenesis (p.76) .
“If a variety were to flourish so as to exceed in numbers the parent species, it would then rank as the species, and the species as the variety; or it might come to supplant and exterminate the parent species; or both might co-exist, and both rank as independent species.”
4) Nevertheless, Darwin does not think that species are real. They are just convenient constructs (p.76).
“From these remarks it will be seen that I look at the term species, as one arbitrarily given for the sake of convenience to a set of individuals closely resembling each other, and that it does not essentially differ from the term variety, which is given to less distinct and more fluctuating forms. The term variety, again, in comparison with mere individual differences, is also applied arbitrarily, and for mere convenience sake.”
UPDATE: John Wilkins does not think that this and similar passages indicate that Darwin considered species as mere conveniences, only that he considered the distinction between varieties (which he called “incipient species”) and species to be mainly arbitrary. He may have a point.
5) Darwin suggests that widespread, numerous species are likely to produce more daughter species (p.77). This would seem to contradict later models of speciation involving geographic isolation, especially ones based on drift in small isolates.
“Hence it is the most flourishing, or, as they may be called, the dominant species,— those which range widely over the world, are the most diffused in their own country, and are the most numerous in individuals,—which oftenest produce well-marked varieties, or, as I consider them, incipient species.”
6) Darwin has compiled a series of data (he doesn’t show them here) comparing genera that are diverse (lots of species) and those that are not, and argues that larger genera include species that themselves include more varieties. He argues on this basis that dominant lineages will become more dominant, since these varieties are incipient species. However, he also notes that this does not continue indefinitely because some previously dominant lineages disappear and some small genera can expand.