What are Bacterial Species?

Abstract

MLSTs 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.

Species concepts and speciation factors in cyanobacteria, with connection to the problems of diversity and classification

“…Bacteria and Archaea are evolutionarily intriguing as they are asexual, and possess extensive populations with relatively short generation times (for review see Cohan 2001Cohan , 2002. Cohesive or disruptive forces shaping bacterial species have remained enigmatic for a long time.…”

“…The ecotype model of prokaryotic speciation proposed by Cohan (2001Cohan ( , 2002Cohan ( , 2006) and Cohan and Perry (2007) treat bacteria as asexual clones, where homologous recombination rates are low. Thus, many crucial questions still remain as unanswered concerning the mechanism of bacterial speciation.…”

Benthic cyanobacteria of the genus Nodularia are non-toxic, without gas vacuoles, able to glide and genetically more diverse than planktonic Nodularia

“…A species should form a phylogenetic cluster and comprise ecologically unique bacteria (Palys et al, 1997;Cohan, 2002). Gas vacuole protein A gene (gvpA), encoding a basic structural protein of gas vacuoles, is often present as multiple copies in cyanobacteria (Barker et al, 1999).…”

Size doesn’t matter: towards a more inclusive philosophy of biology

“…Other microbiologists emphasize the importance of ecological forces on populations, with 'ecotypes' (equivalent to strains) being the product of ecological (but not reproductive) divergence (Cohan 2002;Palys et al 1997;Gevers et al 2005).…”

“…This crude measure of genomic distance is commonly considered equivalent to 97% rRNA identity. The first value was chosen because it appeared to map onto phenotypic clusters for no known evolutionary reasons; the second because it conveniently mapped onto the 70% measure (Cohan 2002;Lan and Reeves 2001). As well as the fact both measures ignore apparently important genomic differences, there is no evolutionary reason why 70% DNA-DNA similarity values should be a species boundary, nor for 16S genes to be exempt from transfer or recombination (Boucher et al 2001;Lan and Reeves 2001;Palys et al 1997).…”