Saturday, February 15, 2014

On the difference between Neutral Theory and random genetic drift

PZ Myers posted an interesting article on The state of modern evolutionary theory may not be what you think it is. He makes the point that there's more to evolution than natural selection.

I think this is an important point but I would not explain it the same way as PZ. He focuses attention on Neutral Theory and the fact that neutral, or nearly neutral, mutations are fixed by random genetic drift. Here's how he describes it ...

First thing you have to know: the revolution is over. Neutral and nearly neutral theory won. The neutral theory states that most of the variation found in evolutionary lineages is a product of random genetic drift. Nearly neutral theory is an expansion of that idea that basically says that even slightly advantageous or deleterious mutations will escape selection — they’ll be overwhelmed by effects dependent on population size. This does not in any way imply that selection is unimportant, but only that most molecular differences will not be a product of adaptive, selective changes.
The debate over adaptationism is a debate over mechanisms of evolution. Random genetic drift is a mechanism of evolution that results in fixation or elimination of alleles independently of natural selection. If there was no such thing as neutral mutations then random genetic drift would still be an important mechanism.

Let's say you have a clearly beneficial mutation with a huge selection coefficient of 0.1 (s = 0.1). Population genetics tells us that the probability of fixation is 2s or, in this case, 20%. That means that the allele will be eliminated from the population 80% of the time. That's random genetic drift. Similarly, some fairly deleterious mutations can sometimes be fixed by random genetic drift.

Random genetic drift is a mechanism of evolution that was discovered and described over 30 years before Neutral Theory came on the scene.

What Neutral Theory tells us is that a huge number of mutations are neutral and there are far more neutral mutations fixed by random genetic drift that there are beneficial mutations fixed by natural selection. The conclusion is inescapable. Random genetic drift is, by far, the dominant mechanism of evolution.

Many people seem to equate Neutral Theory with random genetic drift. They think that random genetic drift is only important when the alleles are neutral (or nearly neutral). Then they use this false equivalency as a way of dismissing random genetic drift because it only deals with "background noise" while natural selection is the mechanism for all the interesting parts of evolution. I think we should work toward correcting this idea by separating the mechanisms of evolution (natural selection, random genetic drift, and others) from the quality of alleles being produced by mutation (beneficial, detrimental, neutral).

The revolution is over and strict Darwinism lost. We now know that random genetic drift is an important mechanism of evolution and there's more to evolution than natural selection. Unfortunately, this blatantly obvious fact is not understood by the vast majority of people and teachers. There are even many scientists who don't understand evolution.