The Modern Synthesis as a Limited Consensus 537
doubts about adaptation in his chapters on variation and change within populations.
No good naturalist, living in our complex universe of relative frequencies,
could ever become an uncompromising dogmatist on the subject of adaptation.
Mayr therefore mentions occasional inadaptive features (1963, p. 156), or
acknowledges the importance of developmental constraint (p. 608). But these
statements function more as footnotes or placeholders in the logic of an argument;
for Mayr does not treat alternatives to adaptation as operational imperatives in the
ordinary analysis of cases. Moreover, Mayr laces his pluralistic admissions with
hedges and caveats. Note, for example, how Mayr frames his main admission of
potential nonadaptation only as an argument against optimality, not as a denial of
selection—and how his closing hedge anticipates a movement of even these least
promising cases into the adaptationist camp:
Each local population is the product of a continuing selection process. By
definition, then, the genotype of each local population has been selected for
the production of a well-adapted phenotype. It does not follow from this
conclusion, however, that every detail of the phenotype is maximally
adaptive. If a given subspecies of ladybird beetles has more spots on the
elytra than another subspecies, it does not necessarily mean that the extra
spots are essential for survival in the range of that subspecies. It merely
means that the genotype that has evolved in this area as the result of
selection develops additional spots on the elytra ... Yet close analysis often
reveals unsuspected adaptive qualities even in minute details of the
phenotype (1963, p. 311).Selection holds primacy of place as the ruling force of evolution: "Every
species is the product of a long history of selection and is thus well adapted to the
environment in which it lives. There is no doubt that the phenotype as a whole,
including its physiological properties, is adaptive and is produced by a genotype
that is the result of natural selection. This is not contradicted by the fact that an
occasional component of the phenotype is adaptively irrelevant" (1963, p. 60).
Above all else, Mayr regards one conclusion as especially well confirmed by
observation: adaptation rules in "every local population" as selection to "exacting
requirements" of local environments produces an "optimal phenotype." One could
hardly state the adaptationist position more boldly: "One conclusion emerges from
these observations more strongly than any other: every local population is very
precisely adjusted in its phenotype to the exacting requirements of the local
environment. This adjustment is the result of a selection of genes producing an
optimal phenotype" (1963, p. 318).
Mayr's treatment of potential alternatives illustrates his adherence to the rule
of adaptation, both as a methodological preference and an empirical claim.
Geographic trends that he formerly attributed to incidental allometries have now
become active adaptations: "A particularly impressive result of studies of
ecogeographical rules is the discovery of the extreme sensitivity of