Bacteria are different: observations, interpretations, speculations, and opinions about the mechanisms of adaptive evolution in prokaryotes.

To some extent, the genetic theory of adaptive evolution in bacteria is a simple extension of that developed for sexually reproducing eukaryotes. In other, fundamental ways, the process of adaptive evolution in bacteria is quantitatively and qualitatively different from that of organisms for which recombination is an integral part of the reproduction process. In this speculative and opinionated discussion, we explore these differences. In particular, we consider (i) how, as a consequence of the low rates of recombination, "ordinary" chromosomal gene evolution in bacteria is different from that in organisms where recombination is frequent and (ii) the fundamental role of the horizontal transmission of genes and accessory genetic elements as sources of variation in bacteria. We conclude with speculations about the evolution of accessory elements and their role in the adaptive evolution of bacteria.