Punctuated Equilibrium and the Validation of Macroevolutionary Theory 883
The theoretical modelling of Allen, Schaffer and Rosko (1993) offers
intriguing support in an implication not discussed by the authors. Allen et al. argue
that the demic structure required for Lieberman's explanation of stasis strongly
buffers species against extinction in chaotic ecological regimes. As an evident
corollary, species selection must favor this architecture for species if such chaotic
circumstances often prevail (or even just occur sporadically enough to impact a
species' fate) over the geographic and temporal ranges of most species in nature.
Thus, stasis would attain a predominant relative frequency among paleospecies
because higher-level selection so strongly favors the persistence of species
composed of multiple, semi-independent demes— the architecture that, as a
consequence, engenders stasis by Lieberman's argument. Allen et al. (1993, p. 229)
write:
Even when chaos is associated with frequent rarity, its consequences to
survival are necessarily deleterious only in the case of species composed of
a single population. Of course, the majority of real world species...
consist of multiple populations weakly coupled by migration, and in this
circumstance chaos can actually reduce the probability of extinction...
Although low densities lead to more frequent extinction at the local level,
the decorrelating effect of chaotic oscillations reduces the degree of
synchrony among populations and thus the likelihood that all are
simultaneously extinguished.NORMALIZING CLADE SELECTION. I cite Williams's (1992, p. 132) term for
what most evolutionists would identify as a form of interdemic selection within
species. (Williams uses "clade selection" as a general descriptor for all forms of
selection among gene pools rather than among genes or gene combinations in
organisms.) Williams also notes, as did Lieberman in a different context, the
paradox of such strong empirical evidence for predominant stasis in the light of
abundant data on substantial change within populations during the geological
eyeblink of human careers in observation and experiment.
Williams therefore proposes, using Bell's work on stickleback fishes as a
paradigm, that the environments of many demes within most species tend to be
highly transient in geological terms, whereas one primary environment (often the
original context of adaptation for the species) often tends to be highly persistent.
(This phenomenon, however well recorded in sticklebacks, need not extend to a
generality for species in nature, as Williams would readily admit in citing
sticklebacks as a paradigm, not a claim for nature's normality. Sticklebacks exhibit
this pattern because they generate successful, but also transient, freshwater demes
from a persisting saltwater stock of lower population density.) Williams (1992, p.
134) therefore argues: "Clade selection acts against freshwater populations either
because they cannot compete in mature freshwater faunas or because their habitats
and ecological niches are ephemeral. The freshwater forms come and go in rapid
succession, but the species complex endures in much the same form for long
periods of time... [based on] the implied rapid extinction and intense clade
selection