Origin Ch.7: Adaptation

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.

In chapter seven, Darwin responds to some more objections to his theory of natural selection. One such objection is that “many characters appear to be of no service whatever to their possessors, and therefore cannot have been influenced through natural selection.” Darwin acknowledges that there may be other processes at play in evolution — he believes that “spontaneous variability” has been under-rated, for example — but he says, “it is impossible to attribute to this cause the innumerable structures which are so well adapted to the habits of life of each species.” The illusion of design can be explained only with the aid of natural selection.

Darwin goes on to explore specific versions of this objection in more detail, and provide explanations for how they can have arisen by natural selection — not mere “just-so stories”, but explanations that can stand up to testing. One such case-study is the apparently random variation in ear and tail size between species of mice: how could natural selection be bothered with these trivial details? These organs turn out to be well endowed with nerves which allow them to act as tactile organs, and the differences in size may therefore be explained as adaptations to the different situations that different species of mice find themselves in.

The life sciences were torn over questions of natural selection and adaptation throughout the second half of the twentieth century, and there are still bitter feelings amongst the veterans. In the 1960s, the likes of George C Williams further developed the principle that natural selection is the only process that can account for the creation of complex traits which appear designed, and introduced methods for identifying adaptations and investigating them. These ideas, and the power of natural selection to explain the design illusion, were popularised by Richard Dawkins, who passionately promoted natural selection at the level of the gene, and argued against other ideas and proposed evolutionary processes which were then popular, such as a group selection.

Since then, other scientists and writers, interested in a variety of other questions in the life sciences, have championed many other processes that are important in evolution. Stephen Jay Gould, for example, was a prominent champion of the role of chance events and accident in the history of life — like the extinction of the dinosaurs allowing the rise of mammals. And there have been huge advances in fields from population genetics to molecular biology and genomics, which have identified new features and processes, most notably genetic drift — random drifting of gene frequencies in populations, unaffected by selection. Gould and geneticist Richard Lewontin also identified a genuine reason why the illusion of design might not always represent adaptation — the famous case of the spandrels, an analogy to an architectural feature that serves no purpose, but simply has to exist as a result of the surrounding features.

I have the benefit of having arrived on the scene after the so-called “Darwin wars” over these issues had died down, and each side had mostly conceded the other’s points. The battles often appear more about ego than substance, and seem to have arisen due to the fact that evolutionary biology is not concerned with just one question, but several. Chance events have become firmly established as important in describing how evolution unfolds over time — in the question of the origin of species. But, with only minor concessions for the likes of spandrels, Darwin’s greatest idea, natural selection, remains the solution to the other great problem: the illusion of design. He was right to defend it so in chapter seven.

Leave a comment

Your email address will not be published. Required fields are marked *