The Modern Synthesis as a Limited Consensus 561
during "normal" times. The theory and factuality of catastrophic mass extinction
has now broken this orthodoxy (see Chapter 12), but simple knowledge of mass
extinction had always posed a threat, especially in Cuvier's original paroxysmal
interpretation (see pp. 484-492). The synthesists therefore treated this apparent
phenomenon in the conventional and congenial way, either by dismissing a
catastrophic cause, or by "spreading out" the time of extinction so that all deaths
could be encompassed by traditional competitive mechanisms, perhaps enhanced
in intensity by rapid environmental changes, and therefore propelling adaptive
evolution even more rigorously. Huxley (1942, p. 446), for example, held that
tough physical conditions only accelerated the competitive takeover by superior
groups: "The worsening of the climate at the end of the Mesozoic reduced the
general adaptiveness of the dinosaurs, pterosaurs, and other reptilian groups, while
increasing that of the early mammals and birds."
Ernst Mayr, in his characteristically forthright way, then linked the denial of
catastrophic extinction, via uniformitarianism, to the crucial second statement in
his definition of the Synthesis (see p. 557), the requirement for extrapolation into
geological vastness: "Yet it has become clear that there is nothing in the past
history of the earth that cannot be interpreted in terms of the processes that are
known to occur in the Recent fauna. There is no need to invoke unknown vital
forces, mutational avalanches, or cosmic catastrophes. Geographic speciation,
adaptation to the available niches (guided by selection), and competition are
largely responsible for the observed phenomena"(1963,p. 617).
In short, by viewing trends as adaptive and anagenetic phenomena, propelled
by competition and building, by a lengthy process of stepwise summation, the
principal pattern of life's history, the Synthesis encompassed the most salient
phenomenon of paleontology within its favored framework of extrapolation. All
causality could reside in the accessible here and now. How then, we must ask, did
the Synthesis treat the two phenomena—speciation and extinction—now viewed as
crucial in breaking the extrapolationist orthodoxy (for if trends must be expressed
as the differential success of some kinds of species vs. others, with most species
formed in geological moments, then the adaptive struggles of populations don't
extrapolate smoothly to changes of mean and modal phenotypes within clades)?
The developing orthodoxy generally acknowledged speciation and then
demoted its importance and distinctiveness. According to Huxley, for example, all
radiations should be treated as adaptive and each event of speciation therefore
represents an independent, gradualistic expression of an anagenetic trend (1942, p.
487): "The adaptive radiation is seen to be the result of a number of gradual
evolutionary trends, each tending to greater specialization—in other words to
greater adaptive efficiency in various mechanisms subservient to some particular
mode of life... Each single adaptive trend also shows the phenomenon of
successional speciation."
In a statement that I find charming, however wrongheaded, Nicholson (1960,
p. 518, at the Chicago centennial symposium) extolled speciation