Punctuated Equilibrium and the Validation of Macroevolutionary Theory 881
so long, that is, as species are free to relocate and 'track' the familiar habitats to
which they are already adapted. Rather than remaining in a single place and
adapting to changing conditions, species move. And so they tend to remain more or
less the same even if the environment keeps on changing."
I place this otherwise conventional explanation towards the heterodoxical end
of my list because habitat tracking embodies the remarkably simple and obvious
(in one sense), yet profound and unconventional view (in another sense) that
evolutionary change represents a last resort, and not a norm for most times, in the
response of populations to their environments. (The second explanation of
plasticity also invokes this theme, but from the organism's, rather than the
population's, perspective.) Habitat tracking also emphasizes the cohesion, and
evolutionary reality, of supraorganismic individuals—an essential theme in the
hierarchical reconstruction of Darwinian theory (see Chapter 8). This subtly
unconventional notion of change as a last resort or default option puts one's mind
in a much more receptive state towards the reality of stasis as a genuine and
fundamental phenomenon in evolutionary theory.
THE NATURE OF SUBDIVIDED POPULATIONS. With this fifth category, we finally
enter the realm of truly—that is, causally—macroevolutionary explanations based
on the reality of supraorganismal individuals as Darwinian agents in processes of
selection. In a brilliant paper that may well become a breakthrough document on
this perplexing subject, Lieberman and Dudgeon (1996) have explained stasis as an
expected response to the action of natural selection upon species subdivided (as
most probably are) into at least transiently semi-autonomous populations, each
adapted (or randomly drifted) to a particular relationship with a habitat in a
subsection of the entire species's geographic range.
Lieberman and Dudgeon derived their ideas (see also McKinney and Allmon,
1995, for interesting support) in the context of Lieberman's extensive multivariate
morphometric analysis of two brachiopod species from the famous Devonian
Hamilton fauna of New York State (see pp. 916-922). Lieberman noted profound
stasis (with much morphological "jiggling" to and fro but no net change) over 6
million years (Lieberman, Brett, and Eldredge, 1994, 1995); but he also studied
samples of each species from each of several paleoenvironments through time.
Paradoxically (at least at first glance), Lieberman documented several cases of
measurable change in single discrete and continuous paleoenvironments through
the section—but not for the entire species integrated over all paleoenvironments
(an argument against habitat tracking, explanation 4 above, as a primary
explanation for stasis). "It was found," Lieberman and Dudgeon write (1996, p.
231), "that more change occurred through time within a single paleoenvironment
than across all paleoenvironments."
Interestingly, such a conclusion also builds a strong argument against the
standard explanation of stabilizing selection (number one of this list) for stasis in
paleospecies—because demes tracking single and stable environments through
time should show no, or at least less, change than the species as a