Species

326 Species

practice, the polyphasic account is not based on a theoretical account of microbial

species, and can be passed over in the context of this paper. Another version of this,

more theoretical and less operational, is the Unified Bacterial Species Concept, in

which the ecological approach to bacteria and other microbes relies on a partial

exchange of genes that are ecologically adaptive:

... a bacterial species is defined as a group of organisms that exploit a common ecology

and, as a result, exhibit effective rates of recombination that are greater among mem-

bers of the group than with other organisms; that is, although interspecific recombina-

tion may occur, the resulting recombinants are, on average, less successful because the

hybrids do not effectively exploit either parental environment.... Over time, the diver-

gence of nucleotide sequences reduces the likelihood of homologous DNA exchange

between lineages, effectively imposing premating genetic isolation.^132

... a microbial species is a concept represented by a group of strains, that contains

freshly isolated strains, stock strains, maintained in vitro for varying periods of time,

and their variants (strains not identical with their parents in all characteristics) which

have in common a set or pattern of correlating stable properties that separates the

group from other groups of strains.^133

In this version, what maintains the homogeneity of clones is that they can exchange

genes that have a crucial role in the adaptive capacity of the organisms. Therefore, it

is a combination of the “biological” concept of species and the ecological concept.

The Quasispecies Model

The third main approach to a natural conception of microbial species (by which
I mean, as opposed to operational, practical, or conventional ones, collectively
called “artificial” conceptions, such as the polyphasic account) is what I will call
the Quasispecies Model, similar to the Phylotype model in intent. According to the
concept developed by Manfred Eigen^134 for viral species, a quasispecies (“as-if” spe-
cies) is a cluster of genomes in a genome space of the dimensionality of the number
of loci. A quasipecies is in effect a cloud of genomes (Figure 13.5) with a “wild-type”
coordinate (that is, the most central genome in the space) that may or may not actu-
ally have an extant or extinct instance.
A genetic cluster of the quasispecies kind is therefore not unlike Mallet’s
Genotypic Cluster Concept,^135 although his is more an operational definition than
a substantive underlying account of species. However, we still need to account for
microbial quasispecies existing in the first place. We have considered one possi-
ble mechanism—gene sharing by lateral transfer—and found it to be insufficient.

(^132) Lawrence 2001, 481.
(^133) Gordon 1978, Rosselló-Mora and Amann 2001, 53.
(^134) Eigen 1993. Eigen and those who have followed him have presented a hypercycle model of self-
organization for why there are quasispecies. However, while this is consonant with the view I present
here, what is important at this point is the phenomenal nature of quasispecies’ genomes—that they
are clusters of genotypes in gene-space—not that they are caused in any general manner offered by
quasispecies researchers.
(^135) Mallet 1995.