The meaning of the neutral theory of evolution

Question

[Ali R. Vahdati]

Hi,

I wonder what does it exactly mean when the neutral theory of molecular evolution predicts that at neutral sites, the rate of substitution is equal to the rate of mutation? I am trying to computationally model this prediction. Does it mean that at each generation, the number of mutations are equal to the number of fixations? or is there some time limit in which the fixation rate will equal the mutation rate?

Many thanks,

Answer 1

[David W]

The per-individual mutation rate is equal to the population substitution rate for neutral variants (not sites, since the same site can have neutral and non-neutral mutations).

If you start with a mono-morphic population it will take a while for the first mutations to fix (the expected time per mutation in 4Ne for a diploid)

[Ali R. Vahdati]

Thanks for your answer. I need some more clarifications. So if I start with a mono-morphic population and if I have a per individual mutation rate of say 0.05, then after 4Ne generations for a diploid population, how many fixations should I expect?

I consider the mutation rate as per individual per generation, is the fixation rate also per generation?

[David W]

If you have forward and backward mutations happening at a rate that high you won't get any fixations -- your population will very quickly equilibrate at a minor allele frequency of 0.5 with mutation creating and destroying minor alleles at an equal rate.

With more realistic mutation rates you probably won't get any mutations fixing in 4Ne generations, as that's the expected time to fixation for a mutation after it turns up (and it takes a while for them to show up). There will almost certainly be a mathematical answer to "the number of mutations expected to fix in a given time, starting from a monomorphic population" out there. But I don't have it to hand.

[Ali R. Vahdati]

Thanks for your reply. I am not looking for any special equation for my question. I want to know what to expect from the definition of fixation rate. When we say that for neutral variants, mutation rate u equals fixation rate k, and we define mutation rate as the number of mutations per individual per generation, and the fixation rate is defined as the number of fixations per population per generation, then we can expect to have Neu mutations happening in the population per generation, where Ne is the effective population size. How many fixation do we expect to happen per generation?

[David W]

I'm not sure what you are asking now, as you seem to have answered you own question: you expect the fixation rate to equal the mutation rate.