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The neutral and nearly neutral theories of molecular evolution 2

Part 2 of 2
Published on July 31, 2022   29 min

A selection of talks on Methods

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0:04
That gives us the main useful principles of neutral theory and brings us to the extension introduced by Tomoko Ohta in 1973, the nearly neutral theory. Let's take a look at that one.
0:19
It's important to remind you what the distribution of fitness effects of mutations were under neutral theory. It's often referred to as an all-or-nothing distribution. You can see that the distribution has quite a high density of strongly deleterious mutations, quite a high density of neutral and some positive. But they're not close to each other. You're either, if you're a mutation in this theory, strongly deleterious or strictly neutral or strongly beneficial. Tomoko Ohta reasoned that this was too unrealistic. She adopts many of the same ideas, we can still see that, on average, because when you make a random change to your protein you're likely to damage its function and reduce fitness. There's still a very high density of negatively selected mutations, and there's still a small occurrence of positively selected mutations because that's the basis of adaptation. She does believe that there is quite a bit of neutral evolution or neutral mutations, but she reasons that if there are neutral mutations and there are deleterious mutations, then some fraction should be slightly deleterious. They would have slightly negative selection coefficients, so she added those to the model. This is basically the first version of the model that was introduced in 1973. She later adds slightly beneficial mutations, and it's the combination of these two that gives us today what we call the nearly neutral theory of evolution. Because these selection coefficients are small, they're either positive but small or negative but small, the intensity of natural selection acting on these mutations is similar in effect size to the intensity of drift, acting on mutations that have s = 0. Because the effect sizes are similar, these two evolutionary processes interact. What Tomoko Ohta is saying is that a large fraction of polymorphism and divergence that we observe at the genetic level in natural settings must result from some interaction between mutation and drift if this distribution of fitness effects is correct.

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The neutral and nearly neutral theories of molecular evolution 2

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