Species

168 Species

Selection is for Darwin not restricted to the intraspecific, but can be interspecific
or even merely a matter of simple survival:

Hence, as more individuals are produced than can possibly survive, there must in every

case be a struggle for existence, either one individual with another of the same species,

or with the individuals of distinct species, or with the physical conditions of life.^46

But the struggle will almost invariably be most severe between the individuals of

the same species, for they frequent the same districts, require the same food, and are

exposed to the same dangers. In the case of varieties of the same species, the strug-

gle will generally be almost equally severe, and we sometimes see the contest soon

decided...^47

And in chapter IV, he notes

In order that any great amount of modification should be effected in a species, a vari-

ety, when once formed, must again, perhaps after a long interval of time, vary or pres-

ent individual differences of the same favourable nature as before; and these must be

again preserved, and so onward, step by step.^48

Darwin had no trouble finding links between asexuals and self-fertilizing species,
and it is clear that he did not exclude asexuals from being species, as we see:

It must have struck most naturalists as a strange anomaly that, both with animals and

plants, some species of the same family and even of the same genus, though agreeing

closely with each other in their whole organisation, are hermaphrodites, and some

unisexual. But if, in fact, all hermaphrodites do occasionally intercross, the difference

between them and unisexual species is, as far as function is concerned, very small.^49

Darwin defined the processes that keep species distinct in even occasionally
sexual organisms as the result of intercrossing. The benefits of sexual reproduction
include “vigour and fertility,” both germane to selection (although he doesn’t explain
exactly why, which resulted in extensive and ongoing debates in the following cen-
tury on the evolutionary benefits of sex). But equally interesting here is that, con-
trary to many twentieth-century Darwinians (for example, Fisher and Simpson^50 ),
Darwin himself has no problem explaining asexual species, and even explains them,
as Manfred Eigen does today,^51 as the result of natural selection entirely.

Intercrossing plays a very important part in nature by keeping the individuals of the

same species, or of the same variety, true and uniform in character. It will obviously

thus act far more efficiently with those animals which unite for each birth; but, as

already stated, we have reason to believe that occasional intercrosses take place with

all animals and plants. Even if these take place only at long intervals of time, the

(^46) Op. cit., 53.
(^47) Op. cit., 60.
(^48) Op. cit., 66.
(^49) Op. cit., 77f.
(^50) See pages 246 et seq.
(^51) Eigen 1993. See also Ereshefsky 2010, Caro-Quintero and Konstantinidis 2012.