Punctuated Equilibrium and the Validation of Macroevolutionary Theory 811
non-causally correlated with coincident evolutionary expressions at other scales;
and (2) in principle, genetic expressions of a common causal structure do not rank
as intrinsically more "deep," "real," "fundamental," or "basic" than other
manifestations in different forms and at different levels; causal relevance depends
upon the questions we ask and the processes that organisms undergo.)
Second, the "allometric" effects of scaling either render the same process in a
very different manner at various scales, or (perhaps more frequently, and primarily
in this case at least) generate the distinctive patterns of different scales by
independent processes, acting simultaneously, but with each process primarily
responsible for results at its own appropriate level.
If I could affirm, as may well often be the case, that punctuated equilibrium
regulated the phenotypic pattern of evolution in a given clade, while genotypic
distances conformed closely to a "molecular clock," I would not conclude that
punctuated equilibrium had therefore been downgraded, or exposed as incorrect,
superficial, or illusory—with genetic continuity as a physically underlying (and
conceptually overarching) reality. Rather, I would regard each result as true and
appropriate for its own scale and realm—with the full pattern of legitimate
difference standing as an intriguing example of resolvable complexity in
evolutionary scaling and causality. Moreover, this particular pattern might easily
result from a highly plausible scenario of complex and multileveled causation—
namely, that neutral substitutions at the nucleotide level impart a signal sufficiently
like a genomic metronome to dominate the molecular results, while ordinary
speciation both regulates the phenotypic history of populations, and works by the
expected pattern of punctuated equilibrium. The genomic results, in principle, need
not extrapolate to encompass the pattern of speciational (macroevolutionary)
change. After all, we do understand that gene trees do not entirely match organism
trees in phylogeny!
In this way—as in the foregoing example of predictable differences between
asexual unicells and sexually reproducing metazoa—punctuated equilibrium
proves its value primarily by hypothesizing sensible distinctions: that is, by
operating at scales and biological conditions where cladogenetic speciation
plausibly sets evolutionary pattern. Punctuated equilibrium should not prevail
where species cannot exist as Darwinian individuals, or where continuously
occurring, and largely nonadaptive, substitution of nucleotides probably regulates
the bulk of genomic change. In this crucial sense, punctuated equilibrium becomes
a valuable hypothesis by delineating such testable distinctions, rather than allowing
evolution to be conceptualized monistically as a single style of alteration, or a
single kind of process either flowing from, or applicable to, all scales of change.
The question of consistency between observed genetic patterns in living
species, and the relative frequency of punctuated equilibrium in their phylogeny,
shall be treated in the next section on the correspondence of punctuated
equilibrium with predictions of evolutionary modeling. But one genetic issue has
been widely discussed in the literature, and should be included in this section