Article

What are Bacterial Species?

Department of Biology, Wesleyan University, Middletown, Connecticut 06459-0170, USA.
Annual Review of Microbiology (Impact Factor: 13.02). 02/2002; 56(1):457-87. DOI: 10.1146/annurev.micro.56.012302.160634
Source: PubMed

ABSTRACT Bacterial systematics has not yet reached a consensus for defining the fundamental unit of biological diversity, the species. The past half-century of bacterial systematics has been characterized by improvements in methods for demarcating species as phenotypic and genetic clusters, but species demarcation has not been guided by a theory-based concept of species. Eukaryote systematists have developed a universal concept of species: A species is a group of organisms whose divergence is capped by a force of cohesion; divergence between different species is irreversible; and different species are ecologically distinct. In the case of bacteria, these universal properties are held not by the named species of systematics but by ecotypes. These are populations of organisms occupying the same ecological niche, whose divergence is purged recurrently by natural selection. These ecotypes can be discovered by several universal sequence-based approaches. These molecular methods suggest that a typical named species contains many ecotypes, each with the universal attributes of species. A named bacterial species is thus more like a genus than a species.

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Available from: Frederick M Cohan, Apr 19, 2014
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    • "Interestingly, around one-third of the OTUs had identity values lower than 95%, half of the OTUs lower than 97% and 2/3 of the OTUs had values lower than 99%. Although the 97% identity is widely used in microbial ecology studies as a broad proxy for 'species' cutoff (Stackebrandt and Goebel, 1994; Cohan, 2002; Hagström et al., 2002), it is well known that this value may integrate different species and overlook putative ecotypes within species with different ecological roles (Fox et al., 1992; Acinas et al., 2004; Stackebrandt, 2006). Therefore, it is safe to assume that we detected at least between 1687 (at 97%) and 2385 (at 99%) putative new prokaryotic OTUs as well as 986 OTUs belonging to putative new genera (at 95%) not present in the standard prokaryotic rRNA gene databases (corresponding to 45.7%, 64.6% and a 26.7% of the total OTUs, respectively). "
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    • "Because of the central position of sex in theories of speciation in eukaryotes, it has been argued that sex is a necessary requirement for species to arise as independently evolving entities (Maynard Smith and Szathmary 1995). According to population genetic theory, however, an asexual population faced with geographical isolation or divergent selection is also expected to diverge into discrete genetic and phenotypic clusters (Templeton 1989; Cohan 2002; Barraclough et al. 2003). These clusters represent independently evolving entities as defined in evolutionary and general lineage concepts of species (Simpson 1951; De Queiroz 2007). "
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    • "Thus, all RD9 deleted strains lack spacers 9 and 39, and its descendants additionally lack spacer 16 and again its descendants also lack spacers 3 and 40–43; moreover , they incorporated information on the host preference, suggesting the concept of ecotypes for the MTBC. According to Cohan (2002), selective sweeps (selection events) might be the cause for limited divergence within groups of bacteria and these groups should be named ecotypes. Through adaptation of a strain to a new niche it becomes immune to periodic selection events in the other clades and therefore has the characteristics of an ecotype. "
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