Allopatric origins of microbial species

Abstract

Although allopatric divergence is a well-accepted mechanism of speciation for eukaryotic macro-organisms, the importance of geographical barriers to divergence in microbial populations is a subject of great debate. Do geographically separated populations of micro-organisms diverge independently, or does their structure fit the often quoted Bass-Becking description 'everything is everywhere; the environment selects'? Aided by high-resolution genetic and genomic tools, the search for 'microbial marsupials' has revealed that in fact both are true; some species of micro-organisms demonstrate allopatric divergence, while others do not. This discovery opens the door for comparative analyses, where questions about the differences in evolutionary and ecological mechanisms that drive divergence and speciation in different microbial species can begin to be explored. Investigating these differences in evolutionary mechanisms will greatly enhance interest in, and understanding of, the dynamic processes that create and maintain the vast diversity of the microbial world.

Figure 1

Two models explain biogeographic differentiation. Both begin with a single homogenous population. (a) Three populations become isolated by geographical barriers and diverge independently. (b) Different genotypes evolve to form a heterogeneous, freely mixing population from which three different environments selected for different types.

Figure 2

Ancestral population structure of Sulfolobus islandicus. Results from Structure analysis show assignment of 47 individuals to five populations associated with five geographical regions. Each vertical bar represents a single S. islandicus strain. Bold lines separate the five different populations, shown in different colours. For each strain, the Y-axis shows the proportion of the variable nucleotide sites in the seven genes that are assigned to each of the five ancestral populations. Stars highlight two individuals from Mutnovsky, for which a significant proportion of the variable sites are assigned to the ancestral Uzon/Geyser Valley population.