The Modern Synthesis as a Limited Consensus 577
yield great insight (for such material obviously represents the only formal contact
that most students ever receive with any given discipline).
I apologize for my almost anecdotal approach, but I think that I have
identified a robust pattern supporting the hypothesis of hardening. I will focus on
the two topics that authors of texts found most congenial in their efforts to explain
synthetic evolutionism to introductory audiences: the centrality of adaptation, and
the sufficiency of synthetic microevolution to explain events at all scales. (I
consider here only the evolution chapters of comprehensive biology texts for
introductory courses, not entire textbooks on evolution. These short, unvarnished
and straight-line accounts of adaptation and extrapolation appear in the context of
such epitomes. Full texts on evolution, which cannot be called "introductory" or
"elementary" (for such courses have always been taught at intermediate or
advanced levels in American universities), do treat the subject more
comprehensively, with a proper listing, often called "textbooky" in our jargon, of
divergent views.)
Adaptation and natural selection
In this age of sound bites, even short chapters include final summaries to tell
students the pith of what they must remember. Consider the following from
Nelson, Robinson, and Boolootian (1967, p. 249), written to summarize a chapter
entitled "Evolution, Evidences and Theories." I cite the entire statement, not an
excerpt:
Principles- Charles Darwin proposed a theory of evolution based on variation,
competition, and consequent natural selection. - The basic mechanism of evolution is now known to be changes in
gene frequencies of populations through time, guided by natural selection.
On the subject of exclusivity, Darling and Darling (1961, p. 199) tell us "any
organism is a bundle of interacting adaptations. Most all the features of all living
things are adaptations." Howells (1959, p. 24), a great evolutionary anthropologist
publishing his popular text in Darwin's centennial year, discussed natural selection
with his usual panache and good humor, but also in the all-encompassing
celebratory mode: "So much for natural selection, the external force, that finger
beckoning to the otherwise unguided heredity of an animal type. All other
principles and facts of evolution may be satisfactorily related to it or explained by
it, and the century following 1859 has seen Darwin triumphant."
Simpson, Pittendrigh and Tiffany (1957, p. 405), an excellent text that
dominated the market for years (and featured a leading architect of the Synthesis as
first author), also stated that any nonrandom evolution must be adaptive: "The
evolutionary changes that result from nonrandom reproduction are clearly
adaptive: the changes are always, necessarily, of such a kind as to improve the
average ability of the population to survive and reproduce in the environments that
they inhabit."