This post is part of a series on The Origin Of Species. It was originally posted on the old blog in feb 2009, during the Darwin 200 celebrations.
Chapter 2 is “variation in nature”. You can’t solve a problem without first defining terms, so if the problem is the origin of species, we’ll be wanting to know just what a “species” is. Turns out, not such a simple question.
In the mid-18th century, Carl Linnaeus founded the modern system of taxonomy, with its hierarchical categories. The Linnaean system is firmly stuck in its time — a time when species were static and discreet, and everything could be neatly packaged into categories. To think in evolutionary terms, one has to unlearn that pre-evolutionary way of thinking, and chapter 2 is where Darwin does this. In the end, instead of defining “species”, Darwin has merely downgraded the significance of such a concept.
The modern biologist knows evolution as (to oversimplify to an extent that will, I am sure, infuriate a few sensitive readers) what happens when you have variation and selection. Variation in this case refers to differences in individual traits, between individual organisms. This variation is the raw material for selection. One of the great products of evolution, meanwhile, is biodiversity — endless forms most beautiful and most wonderful. But in this chapter, Darwin points out that variation and biodiversity are on a continuum. Variety becomes diversity. Separate species are related, with a long line of intermediate varieties; they are only “separate” because some of the historical intermediate varieties have died out.
From Darwin’s thinking and words in this chapter, one can begin to see his gradualist take on evolution. Over great lengths of time, by the introduction of new variation and the extinction of old, varieties may become sub-species, and these become full species, and the same processes account for all the diversity of tangled bank. One can sense that Darwin is prepping the readers to show them that the processes which apply at the level of individual variations also apply to diversity. He is preempting the creationist claim that “microevolution”, at the level of variation, does not imply “macroevolution”, at the level of diversity.
Finally, the chapter left me wondering what Darwin’s take on the 20th century “wars” over these same issues — the “unit of selection”, and whether evolutionary processes can be said to apply at higher levels like species — would have been?