neutrality

Charlesworth B 2008 The origin of genomes − not by natural selection? Curr Biol 18:R140-R141.

  • the most important features of genomes, especially of eukaryotes, are the outcomes of evolutionary forces acting in the absence of, or in opposition to, natural selection
  • paradoxically, therefore, the most complex creatures are the least likely to be under the effective control of selection
  • if these features of the genome are slightly deleterious in terms of their effects on the fitnesses of their carriers, then it is much more likely that they will become established in the genomes of species with small, rather than large, effective population sizes
  • the reasoning is ingenious and Lynch has painstakingly accumulated evidence in favour of his interpretations from a mass of literature on genomics and molecular population genetics
  • there is a real question as to whether his explanations are the whole truth, part of the truth, or simply wrong
  • my suspicion is that they are part of the truth
  • Lynch's evidence comes from broad brush comparisons of extremely disparate types of organism
  • his insistence on the importance of Ne is undermined by the fact that models of the maintenance of transposable elements in intergenic regions (where insertions have little direct effects on fitness) show that there is no difficulty in their establishment in very large populations
  • could it be that the invention of regular sexual reproduction made it easier for mobile, initially self-splicing introns to invade the genome in large numbers?
  • this possibility is not explored by Lynch, who resorts (p. 261) to the untestable hypothesis that there was a long period of reduced Ne among ancestral eukaryotes
  • this is getting dangerously close to the adaptationist just-so stories that he ridicules in the final chapter
  • despite his advocacy of the importance of population genetics, use is made of only a limited set of the tools available in modern population genetics
  • recent work using comparisons of between-species divergence and within-species variability to detect departures from neutrality increasingly suggests that much non-coding sequence is under selection
  • I am especially in sympathy with the strong statements in the final "Genomfart" chapter
  • "nothing in evolution makes sense except in the light of population genetics"
  • and with the criticisms of dubious but fashionable concepts such as 'evolvability'
  • rigorous comparative tests of hypotheses about genome evolution will come to be based on careful contrasts of related taxa
  • differing in far fewer features that those used by Lynch
  • at present, there are too few genome sequences of independent pairs of related species to make this feasible on a large enough scale for there to be much statistical power in such independent contrasts