Species Diversity

 
Some of you might recall my series of postings last year on the top Science questions. One of them was What Causes Species Diversity?. This is an important unanswered question in evolutionary biology even if it's conflated with speciation. We don't really have a good handle on what causes speciation.

That doesn't mean that we are completely ignorant. There are several candidates that, singly or in combination, account for much of what we understand about speciation and diversity. I'd like to quote Richard Dawkins from Unweaving the Rainbow since, as an admitted adaptationist, his view carries much more weight than that of a pluralist. (The reason will become apparent.) Here's how Dawkins describes the problem ...

The standard neo-Darwinian view of the evolution of diversity is that a species splits into two when two populations become sufficiently unalike that they can no longer interbreed. Often the populations begin diverging when they chance to be geographically separated. The separation means that they no longer mix their genes sexually and this permits them to evolve in different directions. The divergent evolution might be driven by natural selection (which is likely to push in different direction because of different conditions in the two geographical areas). Or it might consist of random evolutionary drift (since the two populations are not genetically held together by sexual mixing, there is nothing to stop them drifting apart). In either case, when they have evolved sufficiently far apart that they no longer interbreed even if they were geographically united again, they are defined as belonging to separate species.

Either, or both, of the two main mechanisms of evolution—natural selection and random genetic drift—can lead to speciation and diversity.

One could also argue that diversity depends ultimately on mutation. In this case, the main role of natural selection and random genetic drift is to reduce diversity by eliminating unfit and neutral alleles.

This has always been similar to my understanding of speciation and diversity. I was surprised, therefore to learn that one of my colleagues at the University of Toronto, Spencer Barrett, doesn't think random genetic drift plays a role in speciation [see Darwinism at the ROM]. Barrett is one of the featured presenters in a video at the Darwin exhibit at the Royal Ontario Museum.

In a display on the evolution/creation controversy, I copied down the following statement ...

Darwin's Theory of Evolution by Natural Selection is the only scientific explanation for the spectacular diversity of life on Earth.

So, here's the question of the day. Do you agree with that statement? Do you agree that natural selection is the only scientific explanation of diversity? Spencer Barrett seems to agree. Richard Dawkins would not agree. What do you think?¹

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1. If you disagree with the statement then please try and explain why it is featured so prominently in the Darwin exhibit. Is this an example of framing, or ignorance?