polygenic adaptation

Wright SI 2014 Mutationism 2.0: viewing evolution through mutation's lens. Evolution 68:1225-1227.

  • investigators have also tried to establish the extent to which the rate of adaptation is limited by mutation, compared with natural selection acting on standing genetic variation
  • a new mutation arising in a population spreads rapidly causing a "selective sweep" of neutral genetic variation
  • a predominant mode of adaptation in many systems may reflect selection from preexisting (or readily available) variation, or "soft sweeps" rather than new mutations
  • much of this selection on standing variation may be targeted to polygenic traits with complex genetic architecture
  • there is a growing shift in emphasis in the field of molecular evolution toward the quantitative geneticist's view of evolution, which incorporates both selection from standing variation and, once that variation is exhausted, new mutations
  • the main thesis of the book is that ... adaptation rarely acts on preexisting variation but relies on new major mutations
  • he ... fails to show how any process other than selection can explain the evolution of complex adaptations
  • subsequent chapters incorporating evidence from genomics, developmental biology, population genetics, and speciation research are similarly unconvincing
  • strong selection on phenotypes may often not translate into a strong genomic signal
  • this does not imply the neutrality of the traits in question
  • evolution cannot occur without mutation
  • it can proceed without selection
  • the same cannot be said for adaptation
  • I fail to identify any example in the book of the evolution of adaptation without the action of natural selection
  • mutation rates are typically low
  • population sizes of species are often large
  • only selection can cause allele frequencies and trait means to change over short timescales
  • Nei's viewpoint is essentially a philosophical position, which puts the source of the raw materials (mutation) as primary over the agent that drives the evolution of a highly functioning organism (selection)
  • in Bayesian terms, Nei's prior probability is strongly in favor of mutationism
  • the evidence for natural selection is seen as too weak for the posterior to change from his prior
  • the exciting challenge over the next decade will be to improve our understanding of rates of adaptation
  • including the relative role of selection on new mutations versus standing variation
  • the role of small population size in limiting adaptation
  • the extent to which the rate of adaptation is governed by mutation