Downloaded from http://cshperspectives.cshlp.org/ on September 30, 2021 - Published by Cold Spring Harbor Laboratory Press Realizing Microbial Evolution Howard Ochman Department of Integrative Biology, University of Texas, Austin, Texas 78712 Correspondence:
[email protected] Genome sequences have become the new phenotype for microbial evolutionists. The pat- terns of diversity revealed in the first 100 bacterial genomes fostered development of a comprehensive framework that can explain their contents, organization, and evolution. he study of microbes has been at the fore- tion of the several mitochondrial and viral ge- Tfront of research for some time but has nomes over the previous decade. The era of ge- changed considerably over the past century. nomics, particularly bacterial genomics, is What were originally witnessed as agents of commonly viewed to have begun in 1995 with disease became heralded as models for all life the publication of the genome sequence of Hae- forms with the advent of molecular biology. mophilus influenzae (Fleishmann et al. 1995) And this, in turn, led to two of the most notable followed by the Mycoplasma genitalium genome accomplishments in microbial evolution: an sequence 3 months later (Fig. 1A) (Fraser et al. understanding of both their variation and their 1995). relationships at the deepest and shallowest tax- The field of bacterial genomics was actually onomic depths. These two areas expanded— well underway before the appearance of the first motivated largely by polymerase chain reaction genome sequences. Before 1995, it was already (PCR)—into complementary fields: popula- known (1) that bacterial chromosomes are, tion genetics, which analyzes the source and with few exceptions, circular, possessing a single appointment of genetic variation within bacte- replication origin; (2) that most bacterial ge- rial species, and phylogenetics, which arranges nomes comprise a single chromosome but organisms, even those noncultivable, into a that smaller extrachromosomal elements in the molecular tree of life.