The Modern Synthesis as a Limited Consensus 523
of the early Synthesis. One would therefore regard Wright as the man most likely
to speak for the importance of nonadaptation, and against any functionalist
hardening.
In fact, when interviewed late in life, as both Provine and I can attest, Wright
complained bitterly that his views on the evolutionary role of genetic drift had been
consistently misinterpreted (Wright died in 1988 at age 98, sharp as ever to the
very end). Since genetic drift describes stochastic change in gene frequencies by
sampling error, one might assume that Wright had advocated a radically non-
Darwinian approach to evolutionary change by demoting selection and adaptation,
and boosting the importance of accident. But Wright strongly denied such an
interpretation of his views. He argued, with evident justice apparent to anyone who
reads the works of his last thirty years, that his theory of "shifting balance," while
providing an important role for genetic drift, remains strongly adaptationist—
though adaptation generally arises at a level higher than the traditional Darwinian
focus on organisms.
In brief (see p. 555 for a fuller account), Wright asserted that he had invoked
genetic drift primarily as a generator of raw material to fuel an adaptationist
process of interdemic selection. If the founding deme of a new species occupies
one adaptive peak on a complex landscape (to use standard Wrightian imagery),
movement to additional peaks requires genetic drift— for this stochastic process
permits small demes to descend slopes and enter valleys, where selection can then
draw a deme up to another peak. When demes within a single species populate
several peaks, interdemic selection can operate as a powerful mechanism of
adaptation.
Wright therefore (and accurately) depicted his later shifting balance theory as
adaptationist, and as invoking drift only for a source of variation among demes.
But Wright, though estranged in many ways from the developing synthesis (see
Section 4), followed its trend toward increasingly exclusive emphasis upon
adaptation in evolutionary change. The version of shifting balance that Wright
advocated during the last 30 years of his life did not originate by sudden creation,
complete in this final form. Shifting balance emphasized different themes and
arguments in Wright's earlier work, and these articles, written during the pluralistic
phase of the synthesis, granted a much greater role to randomness and
nonadaptation in evolutionary change. In fact, Wright often, and explicitly,
invoked drift as a non-Darwinian agent of change in articles written during the
early pluralistic phase of the synthesis.
Wright presents a striking example of the principle that later recollections
may be inferior, as historical sources, to written testimony from the time in
question. Provine (1986) has catalogued Wright's ambiguities and multiple intents
during the crucial period of 1929-1932. The later selectionist view already stands
in the wings, but most passages of these early articles advocate the
nonadaptationist role for drift that Wright would later reject (and deny he ever
held). Wright wrote in 1931 (p. 158), for example, that shifting balance "originates
new species differing for the most part in nonadaptive respects." In the following
year, he stated (1932): "That evolution involves nonadaptive