The Modern Synthesis as a Limited Consensus 555
drift, or his proposed mode for the operation of drift within his "shifting balance"
theory of evolution.
Most evolutionists of the 1960's viewed genetic drift only as a random force
of evolutionary change—a prime anomaly under adaptationist hardening (or at
least a factor relegated to the marginal role of efficacy only in tiny populations at
the brink of extinction). Since genetic drift bore such a prominent association with
Wright's name (extending to its original designation as the "Sewall Wright effect"
in early days of the Synthesis), such demotion to marginality relegated the author
to a similar fate under hardline adaptationism.
This situation is surely unfair enough in itself, but—and now the irony—
Wright had also participated in the adaptationist hardening of the Synthesis (see
pp. 522-524), and his later interpretation of genetic drift invoked this concept
primarily as an aid to an enlarged style of adaptationism, and not as a contrary
force in evolutionary change (as he had originally argued). So if Wright had tried
to be helpful in the service of orthodoxy, why did he become so misunderstood and
relegated to the sidelines?
Many reasons might explain Wright's fall into limbo, including the opaque
character of his highly mathematical writing and the fact that he had invoked
genetic drift as a nonadaptive force for change in his earlier work (see p. 523). But
the major factor almost surely resides in a failure of evolutionists to understand the
multi-leveled character of his theory—the aspect that allows drift to serve as an
input to an adaptive process. Wright told me (in an interview in 1981) that he
originally intended to call his shifting-balance process the "two-level theory," for
the full process relies on essential components of both organismic and interdemic
sorting. (Wright also told me that he regarded "exclusive focus on individual
selection" as the major error of the Synthesis.)
In Wright's later formulation of "shifting balance," drift enters into a creative
and selective process in the following manner: The founding population of a new
species moves by selection to an adjacent adaptive peak in a larger landscape. The
chief problem for adaptationism then intrudes: this initial peak will probably not
represent the most favored spot in the landscape (for other unoccupied peaks
probably stand higher), but how can daughter demes ever move to these better
places? Valleys of lesser fitness surround this local peak. If evolution always
proceeds towards adaptation, then the initial population must remain stuck on the
first peak forever. But drift allows small groups to enter valleys, and to cross
troughs into areas where selection may then draw populations up to a higher peak.
When expanding populations, by this process, occupy several peaks in the
landscape, a process of interdemic (interpeak) sorting can occur, eventually leading
to a mean increase in adaptation for the species as a whole. Thus, genetic drift does
not operate as a random force against adaptation in Wright's mature theory, but as
a source of variability for fueling a higher-level process of interdemic sorting. In
other words, drift operates as part of a process that enhances adaptation through
higher-level sorting.