Species

The Development of the Philosophy of Species 325

... a monophyletic and genomically coherent cluster of individual organisms that show

a high degree of overall similarity with respect to many independent characteristics,

and is diagnosable by a discriminative phenotypic property.^129

Such definitions are matters of convenience rather than useful theoretical accounts,

however. Bacterial taxonomists, asked about their ideas about the bacterial species, are

caught between Scylla and Charybdis: either they stick to a coherent species definition

without it necessarily being a biological reality, or they visualize bacterial species as

condensed nodes in a cloudy and confluent taxonomic space. The latter view implies

that classification is a frame for the condensed nodes where some isolated internodal

strains must also get a (provisional) place and name. Loosening the 70% rule often

allows a compromise between the two views ...^130

There is no phylogenetic standard for species, genus, or family delineation, nor is

there a “gold standard” to identify bacterial species, and this is the major reason why a

polyphasic approach is valuable.^131

The polyphasic approach uses genotypic and phentotypic techniques to iso-
late bacterial species. Although an operational definition employed in taxonomic

(^129) Rosselló-Mora and Amann 2001.
(^130) Vandamme et al. 1996, 409.
(^131) Loc. cit., 430.
Lateral gene transfer
FIGURE 13.4 Lateral transfer. In the Recombination Model, functional lateral or horizon-
tal transfer (thick horizontal lines) maintain the cohesion of the cluster of genomes.