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 four is supposed to be the big reveal of natural selection, the solution to one of nature’s greatest problem. Trouble is, the concept is so simple that most readers will probably have worked it out for themselves already. That’s not to say that the chapter contains nothing of interesting — or nothing that isn’t interestingly wrong.
In his discussion of variation, way back in chapter two, Darwin described individual variations between close relatives as being on a continuum with varieties within species, species within genera, and biodiversity in general. In chapter four he takes this to its logical conclusion, describing selection acting at multiple levels. These days, selection “events” are individual life-or-death or reproduction-or-non-reproduction choices, but Darwin is happy to also talk about the life-or-death of a species as an issue of natural selection.
For example, Darwin talks about natural selection ensuring that modifications “… shall not be injurious, for if they were so, the species would become extinct.” But the more important condition is that modifications are not injurious such that they prevent the individual surviving, reproducing, and passing on his genes. Darwin knows that this is so: he later says that, “natural selection acts only by the preservation and accumulation of small inheritable modifications, each profitable to the preserved being.” He knows that it is the selection of individual variations that is the creative force in evolution, but he believes he can see analogous “selection events” in the extinctions of groups, species, and even higher taxonomic levels.
Whether this “selection” of species can be described as a form of natural selection is still an issue to this day. It is obviously true that extinction events can be influenced by the collective inherited variation in the species, and that they will in turn affect the diversity of life, but we generally do not think of them as being selection events, and it’s often easy to dismiss their importance in evolution. This is mainly because of the gene’s eye view of evolution, dominant since the 1960s, which explains why such events are not a productive force in evolution. It seems convenient, therefore, to set aside such events as mere contingency in the history of life, of some interest to paleontologists, perhaps, but not so much for biologists. And, well, the great triumph of Darwin was to demonstrate the powerful and productive force that could account for the illusion of design in life.