The Modern Synthesis as a Limited Consensus 539
exerts a continuous selection pressure on the localized demes of every species and
models them thereby into adaptedness" (1963, pp. 311-312).
Throughout Mayr's 1963 book—with a cadence that sounds, at times, almost
like a morality play—phenomenon after phenomenon falls to the explanatory unity
of adaptation, as the light of nature's truth expands into previous darkness: non-
genetic variation (p. 139), homeostasis (pp. 57, 61), prevention of hybridization (p.
109). Former standard bearers of the opposition fall into disarray, finally
succumbing to defeat almost by definition: "It is now evident that the term 'drift'
was ill-chosen and that all or virtually all of the cases listed in the literature as
'evolutionary change due to genetic drift' are to be interpreted in terms of selection"
(p. 214). All particular Goliaths have been slain (although later genetic studies
would revivify this particular old warrior): "The human blood-group genes have in
the past been held up as an exemplary case of 'neutral genes,' that is, genes of no
selective significance. This assumption has now been thoroughly disproved" (p.
161).
However, Mayr's most interesting expression of movement towards a
hardened adaptationism occurs not so much in these explicit claims for near
ubiquity, but even more forcefully in the subtle redefinition of all evolutionary
problems as issues in adaptation. The very meaning of terms, questions, groupings
and weights of phenomena, now enter evolutionary discourse under adaptationist
presumptions. Not only have alternatives to adaptation been routed on an objective
playing field, Mayr claims in 1963, but the conceptual space of evolutionary
inquiry has also become so reconfigured that hardly any room (or even language)
remains for considering, or even formulating, a potential way to consider answers
outside an adaptationist framework.
Major subjects, the origin of evolutionary novelty for example, now reside
exclusively within an adaptationist framework by purely functional definition: "We
may begin by defining evolutionary novelty as any newly acquired structure or
property that permits the performance of a new function, which, in turn, will open
a new adaptive zone" (p. 602). In a world of rapid and precise adaptation,
morphological similarity between distantly related groups can only arise through
convergence imposed by similar adaptive regimes upon fundamentally different
genetic material. The older, internalist view (constraint-based and potentially
nonadaptationist)—the claim that we might attribute such similarities to
parallelism produced by homologous genes—is dismissed as both old-fashioned
and wrong-headed. (In modern hindsight, this claim provides a particularly
compelling example of how hardened adaptationism can suppress interesting
questions—for such homologues have now been found in abundance. Their
discovery ranks as one of the most important events in modern evolutionary
science—see Chapter 10, p. 1092, where we will revisit this particular Mayrian
claim): "In the early days of Mendelism there was much search for homologous
genes that would account for such similarities. Much that has been learned about
gene physiology makes it evident that the search for homologous genes is quite
futile except in very close relatives" (1963, p. 609).