Microbial diversity – insights from population genetics

Summary Although many environmental microbial populations are large and genetically diverse, both the level of diversity and the extent to which it is ecologically relevant remain enigmatic. Because the effective (or long‐term) population size, N e , is one of the parameters that determines population genetic diversity, tests and simulations that assume selectively neutral mutations may help to identify the processes that have shaped microbial diversity. Using ecologically important genes, tests of selective neutrality suggest that adaptive as well as non‐adaptive types of selection act and that departure from neutrality may be widespread or restricted to small groups of genotypes. Population genetic simulations using population sizes between 10 3 and 10 7 suggest extremely high levels of microbial diversity in environments that sustain large populations. However, census and effective population sizes may differ considerably, and because we know nothing of the evolutionary history of environmental microbial populations, we al