Prokaryote Systematics: The Evolution of a Science

Prokaryote Systematics: The Evolution of a Science: "The "evolutionary clock" (Kimura, 1983) is one of the great discoveries of the 20th century: The fact that in different organisms different (but clearly related) molecular sequences correspond to what appears to be the very same molecular function implies that most of the (net) changes that become fixed over time in any given molecular sequence are selectively neutral; they are of no phenotypic consequence (Kimura, 1983). Such changes must happen more or less randomly in time, and so can be used to measure time in a relative sense. In other words, on the genotypic level a more-or-less steady pace of evolutionary change occurs that is quasi-independent of the sporadic �real� evolutionary changes happening in the overlying phenotype. This independent �evolutionary clock� embedded in the genotype gives the biologist the capacity to infer evolutionary histories and relationships (Woese, 1987); and it has also freed the bacteriologist from the phenotypic quagmire of ill-defined, confusing, or conflicting, and generally phylogenetically uninterpretable morphological and physiological characters."