population expansion
Charlesworth J & Eyre-Walker A 2007 The other side of the nearly neutral theory, evidence of slightly advantageous back-mutations. PNAS 104:16992-16997.
- when there are both slightly deleterious and advantageous back-mutations, there is likely to be an increase in the rate of evolution after a population size expansion
- this increase in the rate of evolution is short-lived
- if there is an expansion in effective population size, there will be a temporary increase in the rate of substitution because of the fixation of advantageous mutations that were previously effectively neutral
- Takano-Shimizu (39) has shown theoretically that this increase in the rate of substitution occurs in a model in which there are slightly deleterious and slightly advantageous back-mutations
- there is an instantaneous increase in the rate of substitution after an increase in effective population size
- this increase can be substantial
- it is relatively short-lived
- we investigate a simple two-allele model
- the population size is assumed to have been constant for a long time
- the system is at statistical equilibrium with respect to mutation, selection, and genetic drift before there is an instantaneous change to a new effective population size
- rate of evolution
- the rate of evolution increases if the population size decreases
- selection becomes less effective and more mutations therefore become effectively neutral
- if there is an increase in population size, but it is less <2-fold, then the rate of evolution decreases, as Takano-Shimizu (39) previously demonstrated
- an increase in population size has two effects
- the fixation probability of deleterious mutations decreases
- the fixation probability of advantageous mutations increases
- for small increases, the change in fixation probability is greater for the deleterious mutations
- hence, the rate of substitution slows
- if the increase in population size is >2-fold, then there is an increase in the rate of evolution
- the increase in the substitution rate is due to the fixation of slightly advantageous back-mutations
- the rate of evolution slows as these get fixed
- there is little evidence of widespread adaptive evolution in hominids (53, 54)
- a question of great interest is how much of the adaptive evolution we detect is due to back-mutation and how much is in response to changes in the environment