The rate and pattern of cladogenesis in microbes.

Theories of macroevolution rarely have been extended to include microbes; however, because microbes represent the most ancient and diverse assemblage of organismal diversity, such oversight limits our understanding of evolutionary history. Our analysis of phylogenetic trees for microbes suggests that macroevolution may differ between prokaryotes and both micro- and macroeukaryotes (mainly plants and animals). Phylogenetic trees inferred for prokaryotes and some microbial eukaryotes conformed to expectations assuming a constant rate of cladogenesis over time and among lineages: nevertheless, microbial eukaryote trees exhibited more variation in rates of cladogenesis than prokaryote trees. We hypothesize that the contrast of macroevolutionary dynamics between prokaryotes and many eukaryotes is due, at least in part, to differences in the prevalence of lateral gene transfer (LGT) between the two groups. Inheritance is predominantly, if not wholly, vertical within eukaryotes, a feature that allows for the emergence and maintenance of heritable variation among lineages. By contrast, frequent LGT in prokaryotes may ameliorate heritable variation in rate of cladogenesis resulting from the emergence of key innovations; thus, the inferred difference in macroevolution might reflect exclusivity of key innovations in eukaryotes and their promiscuous nature in prokaryotes.