Bacterial evolution

Abstract

Bacterial evolution is a topic that 10 years ago was little more than pseudoscience. Today, because of the capacity to determine and compare macromolecular sequences, the study of bacterial evolution is transforming the science of microbiology. Many hundreds of bacterial species have been characterized in terms of partial or complete ribosomal ribonucleic acid sequences, which permit the construction of a comprehensive phylogenetic tree. This phylogeny is of obvious value to the evolutionary biologist. It not only provides a framework within which to understand the evolution of bacterial phenotypes and the general nature of evolutionary process in the microbial world, but also allows one to approach the problems of cellular evolution and the relationship between physical changes in the planet and the evolutionary responses to them. The tree also has considerable predictive and organizational value for microbiology as a whole, affecting and changing the ways in which microbiologists interpret and design experiments as well as their overall perception of the field and its relationship to the other biological disciplines. The bacteria in actuality comprise two separate primary kingdoms, the eubacteria and the archaebacteria, no more related to one another than to eucaryotes. The eubacteria constitute about 10 major groups (the equivalent of eucaryotic phyla); the archaebacteria consist of at least five. The two kingdoms are very different, not only in general molecular phenotype but in the ways in which they evolve. The phylogenetic distribution of phenotypes in both kingdoms suggest that all procaryotes have ultimately arisen from thermophilic ancestry. There seems to exist a clear relationship between the rate at which bacteria evolve and the type of evolution they undergo. A rapid evolutionary course results in a highly altered phenotype, in a sense a damaged phenotype. Both this rate and the accompanying quality of evolutionary change seem to be mirrored in the changes that occur in ribosomal ribonucleic acids and presumably other molecular chronometers.