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 s imple extension of that developed for sexually reproducing eukaryotes. In o ther, fundamental ways, the process of adaptive evolution in bacteria is qu antitatively and qualitatively different from that of organisms for which r ecombination is an integral part of the reproduction process. In this specu lative and opinionated discussion, we explore these differences. In particu lar. we consider (i) how, as a consequence of the low rates of recombinatio n, "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 e volution of accessory elements and their role in the adaptive evolution of bacteria.