The Modern Synthesis as a Limited Consensus 529
would become relatively unimportant and would have minor value to the
study of evolution as a whole (1944, p. 97).To explain these discontinuities, Simpson relied, in part, upon the classical
argument of an imperfect fossil record. But he also conceded that such a prominent
pattern could not be interpreted as entirely artificial—and he recognized that his
favored process of gradualistic Darwinian selection in the phyletic mode would not
provide a full explanation. He therefore proposed his book's only major departure
from explanations based upon selection leading directly to adaptation—and thus, in
his most striking and original contribution, framed the hypothesis of quantum
evolution.
Simpson clearly took great pride in this novel theory, for he ended his book
with a twelve-page defense of quantum evolution, identified as "perhaps the most
important outcome of this investigation, but also the most controversial and
hypothetical" (p. 206). Faced with the prospect of abandoning strict selection in the
gradual, phyletic mode, he framed a hypothesis that adhered rigidly to his more
important goal—the proviso that macroevolution must be rendered by genetical
models and mechanisms operating within species, and amenable to study in living
populations. Thus, he focused upon the only major phenomenon in the literature of
population genetics that permitted a mechanism other than selection to serve as a
basis for directional change— Sewall Wright's genetic drift.
He envisaged major transitions as occurring within small populations (where
drift might be effective and preservation in the fossil record virtually
inconceivable). He chose the phrase "quantum evolution" because he conceived the
process as an "all-or-none reaction" (p. 199) propelling a small population across
an "inadaptive phase"—explicitly so named—from one stable adaptive peak to
another. Since selection could not initiate this departure from an ancestral peak, he
called upon drift to carry the population into an unstable intermediary position,
where it must either die, retreat, or be drawn rapidly by selection to a new stable
position. Simpson felt that, with quantum evolution, he had carried his consistency
argument to completion by showing that genetical models could encompass the
most resistant and mysterious of all evolutionary events—the rapid origin of novel
phenotypes at high taxonomic levels. Quantum evolution, he wrote, is "believed to
be the dominant and most essential process in the origin of taxonomic units of
relatively high rank, such as families, orders, and classes. It is believed to include
circumstances that explain the mystery that hovers over the origins of such major
groups" (p. 206). Simpson could, therefore, conclude: "The materials for evolution
and the factors inducing and directing it are also believed to be the same at all
levels and to differ in mega-evolution only in combination and in intensity" (p.
124).
Simpson's emphasis on quantum evolution underscores a central feature of his
explanatory preferences in 1944—his pluralistic view of evolutionary mechanisms.
He wished to render all of macroevolution as the potential consequence of
microevolutionary processes, not to rely dogmatically upon any