Species

256 Species

Some part of the resource space together with whatever predation and parasitism

occurs on the group considered.^5

Quercus species are often sympatric, and freely hybridize, and yet they maintain

their identity. He refers to these groups as “multispecies,” as they exchange genes,

but he does not require that they actually form viable hybrids that breed true there-

after. “Multispecies” is defined as a

... set of broadly sympatric species that exchange genes in nature ...^6

and refers to the syngameon concept of Verne Grant, which, we have seen, is due to

Poulton. Multispecies can occur without having component species as such, citing

Rubus, Crataegus, and dandelions. The latter leads to the implication that asexuals

are not to be considered species.

Elsewhere Littlejohn has comprehensively reviewed reproductive isolation, and

he makes some interesting comparisons between sexual reproduction and asexual

isolation.^7 He notes that asexuals (uniparental species; unlike many of his predeces-

sors, Littlejohn does not exclude them from specieshood) must be seen as a “clus-

ter or cloud of individuals representing an adaptive node or adaptive peak,” and

cites Dobzhansky and G. E. Hutchinson. Hutchinson employs a “taxonomic space”

model, and treats species as clusters in that space, formed through adaptation.^8 This

is somewhat different to the phenetic concept of an operational taxonomic unit

(OTU). For a start, he requires independence of the axes of the space, and that they

be adaptive characters. Asexuals, such as bdelloid rotifers, are just as good species

as their close sexual relatives, the Nebalia class of crustaceans.^9 Similar points about

the role of adaptation in delimiting species were previously made by entomologist

R. S. Bigelow, who noted that

Reproductive isolation [in Mayr’s 1963 definition—JSW] should be considered in

terms of gene flow, and not in terms of interbreeding, since selection will inhibit gene

flow between well-integrated gene pools despite interbreeding.^10

Recently, the philosopher Kim Sterelny defended an “ecological mosaic” con-
ception of species,^11 based on a reworking of Dobzhansky’s metaphor of species
as occupying “adaptive peaks” in a Wrightean fitness landscape.^12 Noting that spe-
cies are typically ecologically fractured,^13 Sterelny concludes that most species are
geological and ecological mosaics, and are not ecologically cohesive entities. He
takes this to explain stasis in the duration of species, because interbreeding between

(^5) Op. cit., 234.
(^6) Op. cit., 235.
(^7) Littlejohn 1981.
(^8) Hutchinson 1968.
(^9) I am indebted to Dr. Littlejohn for providing me with these references.
(^10) Bigelow 1965, 458.
(^11) Sterelny 1999.
(^12) Dobzhansky 1937, 9–10.
(^13) Sterelny 1999, 124.