330 Species
about speciation and an earlier paper on species concepts,^136 where I employed the
phylogeny of sex to argue that being a biospecies is an evolved trait, not a natural
kind, we ought to expect that each species and group of species has its own unique
evolutionary history and therefore properties, just as limbs and lungs and livers do.
In effect, the modality of “being a species” is an evolved property. There will be a
more general theoretical context of adaptive landscapes, gene dynamics, and so on,
but given that each evolutionary group has encountered different conditions, this
means that each modality will be shared in a fairly limited way.
Moreover, this implies that biospecies is not the most basal notion of species. In
fact, reproductive isolation conceptions of species in general are based on derived
modalities. The basal notion is quasispecies, as all species are at least quasispe-
cies; some are in addition reproductive cohesion or isolation species. Hence some
standard requirements for biospecies, such as reciprocal monophyly, do not need to
apply to all species. But there is one rather interesting aspect to this approach that
I find illuminating: a simple quasispecies is cohered by more or less one adaptive
peak, while a biospecies famously can be and often is adapted as a generalist or have
polytypic traits for differing adaptive peaks (Figure 13.8 above).
What maintains a sexual species in the polytypy case is therefore the combination
of extrinsic selection and internal or intrinsic cohesion, due to selection for repro-
ductive compatibility with potential mates. “Microbial” species, on the other hand,
will tend to depend on adaptive peak cohesion inversely to the degree that they do
exchange their genes, and directly to what functional value of those genes they share
have ecologically.
“Microbial” species form a continuum of gene exchange from 0% to a possible
50% in the case of obligately sexual microbes, and as that percentage drops, the
explanation for the cohesion of homogeneous quasispecies clusters is increas-
ingly ecological niche adaptation, in the absence of stochastic explanations. As it
approaches 50% exchange, the explanation must also include some role for the coad-
aptation of mating types and the lack of fitness of interspecific hybrids. All species
are quasispecies first, and sexual species last, as it were.
(^136) Wilkins 2003, 2007.
Asexual quasispecies Sexual species
A Local fitness peak B Local fitness peaks
Figure 13.8 Asexual and sexual species and niche selection.
maintained by niche selection (A), while pure sexuals are maintained also by reproductive
cohesion (B), allowing multiple fitness peaks to be tracked to a degree.