A reader alerted me to a paper that was just published in BMC Biology.1 The author is Eugene Koonin. He makes the case for neutral evolution (random genetic drift) and against adaptationism. You may not agree with his take on evolutionary theory but you better be aware of it if you claim to be knowledgeable about evolution.
Koonin, E.V. (2016) Splendor and misery of adaptation, or the importance of neutral null for understanding evolution. BMC biology, 14:114. [doi: 10.1186/s12915-016-0338-2]
The study of any biological features, including genomic sequences, typically revolves around the question: what is this for? However, population genetic theory, combined with the data of comparative genomics, clearly indicates that such a “pan-adaptationist” approach is a fallacy. The proper question is: how has this sequence evolved? And the proper null hypothesis posits that it is a result of neutral evolution: that is, it survives by sheer chance provided that it is not deleterious enough to be efficiently purged by purifying selection. To claim adaptation, the neutral null has to be falsified. The adaptationist fallacy can be costly, inducing biologists to relentlessly seek function where there is none.While explaining the limits of natural selection in smaller populations, Koonin makes the point that "Nothing in evolution makes sense except in the light of population genetics" (Lynch, 2006). He points out that,
... although most biologists do not pay much attention to population genetic theory, the time seems to have come for this to change because, with advances in functional genomics, such theory becomes directly relevant for many directions of experimental research.This is important. Evolutionary biologists cannot continue to ignore the fundamentals of evolutionary theory developed by population geneticists over the past century. As we saw in London last November, there are too many so-called "prominent" evolutionary biologists who are ignorant of modern evolutionary theory and the null hypothesis. Ironically, these same biologists are the ones leading the charge for changes in evolutionary theory!
Koonin also makes an interesting point about species with historically small population sizes. These species accumulate a lot of junk in their genomes because natural selection is powerless to remove it. This junk DNA provides a pool of potential exaptations that usually lead to an increase in complexity (constructive neutral evolution). Thus, paradoxically, a weakening of natural selection often leads to more complexity.
A typical example involves gene duplications. In prokaryotes, with large population sizes, the duplicated gene is often eliminated by negative selection because of the slightly deleterious effect of having an extra gene in the genome. In mammals, however, that deleterious effect doesn't meet the threshold for selection and the gene copies persist in the genome. Over time they may acquire independent functions by specialization and now both genes are essential. Thus, the organism has become more complex because it now needs a gene family whereas only one gene was required in species with larger population sizes.
You MUST read this paper in order to understand the real debate within evolutionary theory. It's a very good summary of the point of view I've been trying to communicate for a very long time. I'm just a messenger but Koonin is one of the players.
1. I don't have permission to reveal their identity.