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