Species

210 Species

... we have some reason to suppose that they [allelomorphic loci, or alleles in modern

parlance—JSW] form a very small minority of all the loci, and that the great major-

ity exhibit, within the species, substantially that complete uniformity, which has been

shown to be necessary, if full advantage is to be taken of the chances of favourable

mutations.^4

Fisher is here dealing with the existence of asexual species, as they present a
problem for him, or rather, they would have if he had been (at that time) certain that
any organisms existed without any sexual reproduction (the claim was not revised in
the 1958 edition). In this case, he says:

In such an asexual group, systematic classification would not be impossible, for groups

of related forms would exist which had arisen by divergence from a common ancestor.

Species, properly speaking, we could scarcely be expected to find, for each individual

genotype would have an equal right to be regarded as specifically distinct. And no nat-

ural groups would exist bound together by constant interchange of their germ-plasm.^5

Clearly, this exchange of genes in germ-plasm is the sine qua non of a species

for Fisher. But, he goes on to say, there would be an analogue of species in asexuals:

The groups most nearly corresponding to species would be those adapted to fill so

similar a place in nature that any one individual could replace another, or more explic-

itly that an evolutionary improvement in any one individual threatens the existence of

all the others.

So while Fisher is a realist about asexual groups, they are ecological groups

adapted to the environment in which they find themselves, and kept distinct in vir-

tue of selection against less-fit variants. This resolves also the problem of favorable

mutations—if a novelty of value arises in an asexual lineage then it will not spread

throughout the population, but instead it will replace the population. In sexual organ-

isms that have proper species, selection maintains the identity of populations rather

than of lineages, and favorable mutations can be spread by recombination of genes.

But Fisher does not think that there will be many of these groups, and that if they did

exist they would be those groups “of so simple a character that their genetic constitu-

tion consisted of a single gene.”^6

Of sexual species proper, Fisher presents the view that apart from geographical

isolation, in which

the two separated moieties thereafter evolv[e] as separate species, in almost complete

independence, in somewhat different habitats, until such time as the morphological

differences between them entitle them to ‘specific rank’” [p. 139].

Species are also caused to fission by what we now call sympatric speciation,

because in “many cases it may safely be asserted that no geographic isolation at all

(^4) Loc. cit.
(^5) Fisher 1930, 135.
(^6) Fisher 1930, 137.