540 THE STRUCTURE OF EVOLUTIONARY THEORY
Subjects that might have seemed challenging or exceptional now achieve a
place within the adaptationist framework by expanded definition. Nonfunctional
pleiotropic consequences, for example, become an aspect of orthodoxy because
they now enter a hardened discourse in the redefined guise of features subsidiary to
a main effect of adaptive significance. (I do not challenge the particular assertion
in this case, but I do feel that such an important subject deserves consideration
from a structuralist perspective as well): "Pleiotropic gene action is the key to the
solution of many other puzzling phenomena ... Color, pattern, or some structural
detail may be merely an incidental by-product of a gene maintained in the gene
pool for other physiological properties. The curious evolutionary success of
seemingly insignificant characters now appears in a new light" (1963, p. 162).
All potential anomalies yield to a more complex selectionist scenario, often
presented as a "just-so-story." Why did the crown height of molars increase so
slowly, if hypsodonty became so advantageous once horses shifted to vegetational
regimes of newly evolved grasses with high silica content? Mayr devises a story—
sensible, though empirically wrong in this case—and regards such a hypothetical
claim for plausibility as an adequate reason to affirm a selectionist cause. (The
average increase may have been as small as the figure cited by Mayr, but horses
did not change in anagenetic continuity at constant rates. Horses probably evolved
predominantly by punctuated equilibrium— see Prothero and Shubin, 1989, and
Chapter 9. The average of a millimeter per million years represents a meaningless
amalgam of geological moments of rapid change during speciation mixed with
long periods of stasis): "An increase in tooth length (hypsodonty) was of selective
advantage to primitive horses shifting from browsing to grazing in an increasingly
arid environment. However, such a change in feeding habits required a larger jaw
and stronger jaw muscles, hence a bigger and heavier skull supported by heavier
neck muscles, as well as shifts in the intestinal tract. Too rapid an increase in tooth
length was consequently opposed by selection, and indeed the increase averaged
only about 1 millimeter per million years" (1963, p. 238).
In 1991, I asked Ernst Mayr about changes between his 1942 and 1963 books.
He acknowledged the structural alterations, of course—particularly his addition of
several chapters emphasizing adaptational themes. But he strongly denied any
personal augmentation of adaptationist preferences through the intervening years,
citing the interesting argument that, as a Lamarckian in his evolutionary youth
(well before both books), he had always favored adaptationism. He even wrote me
a fascinating letter the day after our lunchtime conversation:
Dear Steve,
I gave considerable thought to your question how my 1963 book
differed from the 1942 one, and why adaptation was so much more featured
in the later volume. I think I now have the answer.
Remember that I consider evolution by and large to consist of two