130 THE STRUCTURE OF EVOLUTIONARY THEORY
reproductives (p. 236): "How the workers have been rendered sterile is a difficulty;
but not much greater than that of any other striking modification of structure; for it
can be shown that some insects and other articulate animals in a state of nature
occasionally become sterile; and if such insects had been social, and it had been
profitable to the community that a number should have been annually born capable
of work, but incapable of procreation, I can see no very great difficulty in this
being effected by natural selection."
The phrase "profitable to the community" seems to imply group selection but,
as argued above, this modern interpretation need not reflect Darwin's intent. He did
not, after all, know about haplodiploidy, different degrees of relatedness, or parent-
offspring conflict. He does not argue here at the locus classicus for modern
theories of group selection—altruism defined as the rendering of aid (at personal
peril or expense) to non-relatives. Rather, he views the hive as a group of
cooperating bodies, all tightly related and all generated by the queen. Anything
beneficial to the hive fosters the reproductive success of the queen in ordinary
natural selection upon her as an individual. The sterility of a worker does not differ
in principle from the horns of an ox—a trait not found in parents, but produced by
selection on parents. A queen that can generate more sterile workers might be
favored by selection just as a breeder picks cows that yield castrated oxen with
longer horns.
At most, one might hold that Darwin treats the entire hive as an entity—a
statement about higher-level selection on the "superorganism" model (see D. S.
Wilson and Sober, 1989, and Sober and Wilson, 1998). But here we meet an issue
that must be regarded as more linguistic than substantive. Just as Janzen (1977)
wishes to identify a clone as a single El (for "evolutionary individual"), and to treat
single bodies of rotifers or aphids as parts, so too might Darwin view the bodies in
a hive as iterated organs of the whole. Nonetheless, selection acts on the queen as
an individual reproducer. The determinants of her success undoubtedly include the
form and function of her sterile offspring. Natural selection can "get at" a beaver
through the form of its dam, or at a bird through the shape of its nest—and we do
not talk about selection on the higher-level entity of organism plus product. Why
should selection not "get at" the queen ant or bee through the conformation of the
hive and the function of its members? (See Ruse, 1980, for a parallel argument, in
agreement with mine, on Darwin's explanation of hymenopteran castes by
organismic selection.)
Darwin takes up a different challenge to the exclusivity of organismic
selection in the next chapter on "Hybridism." Crosses between varieties of a
species are usually fertile, but crosses between species are generally sterile, or at
least greatly impaired in fecundity. Under the guiding precepts of gradualism and
uniformitarian methodology, we must view species as former varieties promoted
by selection to the greater difference of true distinctness. But natural selection
could not have built sterility in gradual degrees from an original fertility between
parent and offspring—for sterility cannot benefit the hybrid individual: "On the
theory of natural selection the case is especially important, inasmuch as the sterility
of hybrids could not possibly be of any advantage