neutrality

Nei M, Suzuki Y & Nozawa M 2010 The neutral theory of molecular evolution in the genomic era. Annu Rev Genom Hum Genet 11:265-289.

  • at present, genome-wide analyses of natural selection consist of collections of single-locus analyses
  • phenotypic evolution is controlled by the interaction of many genes
  • the study of natural selection ought to take such interactions into account
  • the above definition was a mathematical formality for Kimura
  • he was actually interested in neutral or nearly neutral mutations in the biological sense
  • how can such a small s remain constant for the entire fixation period?
  • for practical purposes the definition of neutrality should roughly be |s| < 0.001 irrespective of population size in vertebrates
  • s is taken to be the mean selection coefficient over evolutionary time
  • the above definition of neutrality is appropriate for directional selection
  • in the case of overdominant selection, even smaller s values may have significant effects
  • in major histocompatibility complex (MHC) loci, where strong overdominant selection operates, the selection coefficient has been estimated to be 0.0007 to 0.042
  • Ohta (128) proposed that this dilemma can be resolved by assuming that most mutations are slightly deleterious and can be fixed in small populations, more easily by genetic drift than in large populations
  • if this argument were correct, the genomes of large organisms would be expected to deteriorate gradually
  • in reality these organisms are more advanced in terms of organismal complexity than small organisms
  • the fast evolutionary rate of animal mitochondrial genes was first thought to be due to Muller's ratchet effect
  • which would enhance the fixation of slightly deleterious mutations in asexual haploid populations because of the lack of recombination
  • this explanation is unsatisfactory because plant mitochondrial genes, which have the same mode of inheritance as animal mitochondrial genes, evolve very slowly
  • the fast evolution of animal mitochondrial genes is due to a higher mutation rate partly attributable to lack of the DNA repair gene RecA
  • which is present in plant mitochondrial genomes