Reproductive Isolation Concepts 233generate species, whatever that level may be. Under both mechanisms, the spread of
genetic variants through populations is the sine qua non of species rank.
As Ghiselin observes,^18 Templeton’s conception resembles Mayr’s in that species
form as the result of genetic revolutions that constrict exchangeability, but he also
favors Paterson’s views on mate recognition. “Casting the definition in terms of one
gene to be exchanged for another makes it roughly the same as the biological species
definition.” The requirement for demographic exchangeability means that organisms
as units can replace each other in genetic populations, even if they do not do so in
actuality.
Mallet has offered up “a species concept for the modern synthesis,” the genotypic
cluster species concept (GCSC) in which species were identified as “identifiable
genotypic clusters.”^19 Although a genotypic notion, it in many ways resembles the
phenetic concept in which instead of morphological variables, the lack of intermedi-
ates lies in single genetic loci and ensembles of multiple loci. Like the biospecies
concept, the GCSC applies only to populations in sympatry or parapatry, and lacks a
nonarbitrary level of similarity or isolation of genetic alleles to specify specieshood.^20
In many ways, it also appears to be a genetic version of the Diagnostic species concept
described in the next chapter. It has not found much use outside of this paper, however.
Wu’s views are harder to pin down exactly; he may not in fact have a distinct
species concept at all.^21 He claims that reproductive isolation (RI) has involved the
entire genome under the BSC, but that it instead involves genetic isolation of adap-
tive genes and gene complexes, in particular, what he calls speciation genes. Even if
there remains gene exchange for the non-adaptive genes, if there is RI for the adaptive
genes, good species have evolved. His view relies on speciation itself, the process,
resulting in some degree of genome isolation. Species are defined in terms of their
genes having reached a particular level of RI, either Stage III—where populations
have diverged sufficiently that they will not fuse in sympatry—or Stage IV—where
populations are entirely isolated and will not mix at all genetically. Speciation genes
have become a topic of interest to researchers with the rise of sympatric speciation
models and examples, but Wu’s genic conception falls within the traditional genetic
and biospecies concepts.^22
In a major review of research into speciation, Coyne and Orr present a revised ver-
sion of the BSC in which they make a number of concessions to criticisms and in which
they defend against some others.^23 Coyne and Orr’s version, which I will here call the
limited BSC, allows for limited introgression, does not insist upon integration of gene
complexes, permits other definitions for asexuals and paraphyly of species, and treats
ecological differentiation as necessary to the persistence of species (in sympatry, at
(^18) Ghiselin 1997, 113.
(^19) Mallet 1995, 296; cf. also Mallet 2000, Dres and Mallet 2002, Beltrán et al. 2002, Boenigk et al.
2012.
(^20) Brower 2002.
(^21) Wu 2001a, 2001b.
(^22) Vogel et al. 1996, Bridle and Ritchie 2001, Butlin and Ritchie 2001, Rieseberg and Burke 2001, Van
Alphen and Seehausen 2001, Orr and Masly 2004. The mode of speciation as a way to identify the
nature of species is the subject of Wilkins 2007.
(^23) Coyne and Orr 2004, chapter 1.