The Population-Genetic Environment

Abstract

Abstract Evolution depends ultimately on the population-genetic environment, defined by mutation and recombination rates and the power of random genetic drift. The latter, which is an inverse function of the effective population size, defines the noise in evolutionary dynamics. If the power of drift exceeds the deterministic force of selection, the latter is rendered ineffective, resulting in patterns of evolution entirely determined by mutational forces. These issues are central to understanding evolutionary processes because all three major forces vary by about four orders of magnitude across the Tree of Life. Moreover, the mutation rate evolves to become inversely related to the effective population size, so populations experiencing greater drift simultaneously experience increased mutational pressure. In addition, average rates of recombination per nucleotide site decline in larger organisms, which evolve larger chromosomes owing to the accumulation of large amounts of non-functional DNA, but still experience just one to two crossover events per chromosome arm during meiosis. This covariation of rates of mutation, recombination, and drift across the Tree of Life influences the ways in which different lineages evolve in response to natural selection, in effect creating drift barriers beyond which selection cannot proceed. In large multicellular organisms with small effective population sizes, natural selection is constrained to utilize mutations with relatively large effects, whereas microbes with large effective population sizes are capable of discriminating mutations with very tiny effects, opening up the capacity for more fine-grained evolution.