What do we mean when we talk about bacterial species? As we saw in Chapter 22, Ernst Mayr defined species as “groups of interbreeding natural populations that are reproductively isolated from other such groups.” This widely accepted definition is called the biological species concept, but a moment’s reflection will show that it can’t be applied to the most abundant organisms on Earth—Bacteria and Archaea. In the absence of sexual reproduction, how do we think about species in the prokaryotic domains?

Some microbiologists argue that we should abandon the concept of species when talking about bacteria, especially since distantly related bacteria can exchange genes. Nonetheless, nature is full of microbial populations with well-defined features of form and function.

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In the age of molecular sequence comparisons, bacterial species have sometimes been defined as populations whose small-subunit rRNA sequences are more than 97% identical. It would be useful, however, to conceive of bacterial species in terms of functional processes, much as Ernst Mayr did for eukaryotic organisms. In plants, animals, and single-celled eukaryotes, interbreeding ensures that populations share the same pool of genes, providing a counterforce to mutations and selection pressures that promote divergence.

A similar process exists in bacteria. In early research on the population genetics of bacteria, biologists observed that genetic diversity in laboratory cultures gradually increases through time and then rapidly decreases with the emergence of a successful variant that outcompetes the rest. The episodic loss of diversity is called periodic selection. Some biologists argue that this process provides a means of recognizing bacterial species: Populations subject to the same episodes of periodic selection belong to a single species. This is a useful way to define bacterial species, but other definitions are possible, and as yet there is no consensus among microbiologists.