community genetics

Wade MJ 2007 The co-evolutionary genetics of ecological communities. Nat Rev Genet 8:185-195.

  • I show how the co-inheritance of trans-specific gene combinations can be estimated using a parameter that was devised to measure epistasis between genes within the same genome
  • evolutionary theories in the mid-1980s established that co-evolution, strictly speaking, does not take place between two species, but rather between the traits of two (or more) species
  • the outcome of natural selection working on a population of a particular species can depend on the standing genetic variation in a population of a different species
  • these considerations have led to the recent confluence of genetics and ecology resulting in the birth of a new field, community genetics
  • population genetic theory that was developed to understand the evolution of gene interactions (epistasis) that underlie complex traits41–43 has recently been applied to host and symbiont co-evolution
  • the role of gene interactions in evolution is difficult to understand because adaptive gene combinations are not transmitted directly from parents to offspring
  • there is a tension between natural selection increasing the frequency of a particular combination and transmission breaking it up
  • population genetic structure prevents complete random mating and mixing of genes across an entire species
  • the parameter Θ measures the degree of co-inheritance of gene combinations
  • when Θ is low, as in the case of unlinked genes in large, randomly mating populations, then gene combinations have little if any heritability
  • selection on them must be strong for any adaptive progress to occur
  • the maintenance of synteny of genes over long evolutionary time periods is indicative of the evolutionary importance of epistasis and high Θ
  • effective population size of mitochondrial genes is one-quarter that of nuclear genes
  • the mitochondrial genome is haploid and inherited only through females in most organisms
  • mitochondrial variants become fixed in populations by random genetic drift more rapidly than do nuclear variants
  • this chance fixation has its own consequences for adaptive evolution when there is epistasis between cytoplasmic and nuclear genes
  • random genetic drift converts epistatic to additive nuclear variation
  • when one genome experiences stronger drift than the other, there is a bias in the conversion of epistatic to additive variance
  • the bias favours the creation of additive nuclear variation, which is the kind that is useful for an adaptive response to natural selection
  • for such cyto-nuclear gene combinations, mutation and drift govern evolution of the mitochondrial genes
  • natural selection governs evolution of the nuclear genes
  • random genetic drift in a two-species system is governed by the species with the smaller effective population size in an analogous way
  • Kiester, A. R., Lande, R. & Schemske, D.W. Models of coevolution and speciation in plants and their pollinators. Am. Nat. 124, 220-243 (1984).
  • these authors established the principles that co-evolution takes place between traits and that the co-evolutionary effective population size is determined by the rarer species