Species

258 Species

it applies equally to uniparental lineages in artificial life simulations as in biology.^20

He notes that

...A viral species ... is actually a complex, self-perpetuating population of diverse,

related entities that act as a whole^21

and he defines the quasispecies as a

... region in sequence space [which] can be visualized as a cloud with a center of grav-

ity at the sequence from which all mutations arose. It is a self-sustaining population of

sequences that reproduce themselves imperfectly but well enough to retain a collective

identity over time.^22

The conception is based on the observation that in a cluster of genotypes of

viruses, there will be a mean genotype (the wild type) maintained by selection for

optimality for that particular environmental niche (Figure 13.6). Eigen noted that

it may eventuate that there is actually no single virus with that “wild type” geno-

type. Clearly, this makes the agamospecies conception a kind of ecotypical notion,

maintained by natural selection; in fact, one might say that it is the purely ecological

aspect of the ecospecies conception. Similar observations were made by Hutchinson,

but no technical name was applied by him at that time.^23 I therefore propose that

“agamospecies” and “quasispecies” be treated as synonyms, and that the inferences

made about quasispecies by Eigen be applied to agamospecies hereafter.

Microbial Species

Microbial species are covered by several differing concepts. Bacteriologist Frederick

Cohan has proposed a bacterial species concept:

[Bacterial species] are populations of organisms occupying the same ecological niche,

whose divergence is purged recurrently by natural selection.^24

The polyphasic species concept has been proposed in which multiple lines of
independent evidence are used:

This polyphasic taxonomy takes into account all available phenotypic and genotypic

data and integrates them in a consensus type of classification, framed in a general phy-

logeny derived from 16S rRNA sequence analysis.^25

(^20) Wilke et al. 2001.
(^21) Eigen 1993b, 32.
(^22) Op. cit., 35.
(^23) Hutchinson 1968, 184.
(^24) Cohan 2001, 2002, 2006.
(^25) Vandamme et al. 1996. See also Varghese et al. 2015.