Species

Other Species Concepts 257

populations within the species’ range acts as an inhibitor, which he calls Mayr’s

Brake, on adaptive change over all the entire species. He holds that this reinforces an

evolutionary concept of species.

Ecological species concepts are generally partial rather than all-encompassing,

and tend to act as adjuncts to more universal conceptions. It is clear that all ecologi-

cal concepts rely heavily on the pre-eminent role of natural selection to maintain

isolation between groups. Selection also plays a critical role in the definition and

delimitation of asexual species, to which, among others, we now turn.

“Aberrant” Concepts

“Aberrant concept” here usually merely signifies that the concept is either not

attended to much by zoologists, or that it is not thought by some biologists to be a

“true” species concept.

Agamospecies

Agamospecies are asexual taxa. The term was defined by Cain as the end result of
parthenogenesis in animals and apomixis in plants (which is secondary asexuality),^14
but it is now well understood that most unisexual organisms are in fact neither^15 — that
is, are primarily asexual, mostly bacteria or algae. Cain defined agamospecies as

... those forms to which [the biological species concept] cannot apply because they

have no true sexual reproduction ...^16

This is clearly unsatisfactory. For a start, it is a privative definition—agamospecies

are what are not something else (in this case taxa comprised of sexual organisms). As

we saw also with Fisher, asexuals are considered by Cain to be off the mainstream of

evolution, something of a marginal occasional misfiring of the evolutionary process,

since sexual recombination permits the more rapid acquisition and spread of favor-

able mutations, and asexual reproduction is subject to genetic load (the continuation

of deleterious mutations). This is no longer the consensus.

So, it has been questioned whether agamospecies are anything more than the

morphological species concept applied to asexual organisms.^17 However, there is a

notion that applies to asexuals that is, I believe, a more coherent way to understand

asexuals, and which avoids privative definition. Originally defined for viruses (which

are mostly, but not always,^18 asexual), the concept of the physicist Manfred von Eigen

is called quasispecies (from the Latin for “as if, just as, as it were... nearly”),^19 and

(^14) Cain 1954, 98–106.
(^15) Kondrashov 1994, Schloegel 1999, Taylor et al. 1999.
(^16) Cain 1954, 103.
(^17) Noted by Dobzhansky 1937, 1941, Sonneborn 1957, 283.
(^18) Viruses can crossover genetic material in a superinfected host [Szathmáry 1992, Boerlijst et al. 1996].
(^19) Eigen 1993a, 1993b, Stadler and Nuño 1994. See entry “quăsĭ” in Lewis and Short 1879.