THE EFFECTS OF RARE BUT PROMISCUOUS GENETIC EXCHANGE ON EVOLUTIONARY DIVERGENCE IN PROKARYOTES

I have investigated the determinants of genetic divergence within a metapopulation of bacterial populations, in which each constituent population is adapted to a different ecological niche and recombination occurs, rarely, within and between populations. Recombination was shown to be unlikely to constrain adaptive divergence between bacterial populations. Under relatively high rates of recombination, populations may diverge in ecological characters without diverging in neutral characters. Under lower recombination rates, populations may diverge in neutral as well as adaptive characters, even if they are not sexually isolated (i.e., if recombination occurs at the same rate within and between populations). Because bacterial populations that exchange genes may diverge in neutral and adaptive characters, extension of the biological species concept to bacteria may be inappropriate. Groups of strains that cluster by DNA sequence may each represent either a single ecological population or a group of ecological populations whose neutral divergence is constrained by recombination. The level of neutral diversity within a metapopulation is determined by a complex interaction among recombination rates, periodic selection rates, population size, and ecological differentiation. Therefore, one may not easily estimate these parameters from measures of nucleotide diversity within a ''species'' if the species is a metapopulation consisting of multiple ecological populations.