The Essence of Darwinism and the Basis of Modern Orthodoxy 131
to them, and therefore could not have been acquired by the continued preservation
of successive profitable degrees of sterility. I hope, however, to be able to show
that sterility is not a specially acquired or endowed quality but is incidental on
other acquired differences" (p. 245).
Darwin considers two possible explanations. He constructs his entire chapter
on hybridism as a defense of natural selection in its ordinary, organismal mode
through the rejection of one explanation based on species selection and the
advocacy of another rooted in selection on organisms with an interesting twist.
Darwin admits that species selection, at first glance, seems to provide a simple and
attractive solution: interspecific sterility must originate as an adaptation of species,
built and promoted to preserve integrity by preventing introgression and
subsequent dissolution. (A. R. Wallace strongly promoted this view. Darwin's firm
rejection led to a protracted argument that strongly colored their relationship—see
Kottler, 1985; Ruse, 1980.)
But Darwin rejected this explanation because he could not conceive how a
species might act as an entity in this manner. Nonetheless, he could not possibly
argue in response that hybrid sterility arose by direct selection for the trait itself.
He therefore proposed a subtle argument, almost surely correct in our current
judgment, for the origin of hybrid sterility as an incidental consequence of other
differences established by organismal selection. A. R. Wallace, in striking contrast,
remained so committed to viewing every natural phenomenon as a direct
adaptation that he willingly roamed up and down among levels of selection (quite
unaware of the logical difficulties thus entailed) until he found a locus that could
support a direct adaptive explanation.
Darwin argued that any population, in diverging far enough from an ancestor
to rank as a separate species, must undergo a series of changes (usually extensive),
mediated by natural selection and leading to a set of unique features. Any two
species will therefore come to differ in a series of traits directly built by natural
selection. These disparities will probably render the two species sufficiently unlike,
particularly in rates and modes of reproduction and development, that any hybrids
between them will probably be stunted or infertile—not because selection acted
directly for sterility, but only as an incidental effect of differences evolved by
natural selection for other reasons. Although interspecific sterility cannot be built
directly by selection for its advantages to organisms, this feature can and will
originate as a consequence of ordinary selection on organisms. Darwin contrasts
his proposal with Wallace's alternative based on direct adaptation via species
selection:
Now do these complex and singular rules indicate that species have been
endowed with sterility simply to prevent their becoming confounded in
nature? I think not. For why should the sterility be so extremely different in
degree, when various species are crossed, all of which we must suppose it
would be equally important to keep from blending together? ...The
foregoing rules and facts, on the other hand, appear to me clearly to
indicate that the sterility both of first crosses and of hybrids is simply