880 THE STRUCTURE OF EVOLUTIONARY THEORY
of species is a tenuous thing indeed. But if that stability is an inherent
property both of individual development and the genetic structure of
populations, then its power is immeasurably enhanced, for the basic
property of homeostatic systems, or steady states, is that they resist change
by self-regulation. That local populations do not differentiate into species,
even though no external bar prevents it, stands as strong testimony to the
inherent stability of species in time.This proposal became one of the most widely controverted aspects of
punctuated equilibrium, especially in linkage with other, largely independent
concepts like the prevalence of neutral change (Kimura, 1968), and the exaptation
of originally nonadaptive spandrels (Gould and Lewontin, 1979), also viewed as
challenges to the more strictly adaptationist concept of Darwinian evolution then
prevalent. I now believe that these criticisms, with respect to the issue of stasis in
paleospecies through geological time, were largely justified—and that the theme of
constraint, while not irrelevant to the causes of stasis in punctuated equilibrium,
does not play the strong role that I initially advocated. (However—and perverse as
this may seem to some detractors— my conviction about the general importance of
constraint vs. adaptationism at other more appropriate scales has only intensified,
particularly in the context of revolutionary findings in developmental genetics—
see Chapter 10.)
I have changed my initial view for two primary reasons. First, the arguments
of Mayr and Lerner, the intellectual underpinnings of our initial proposals about
constraint, have not held up well under further scrutiny, particularly in the
privileging of small populations as especially, if not uniquely, endowed with
properties that permit the breaking of stasis. Further modelling has led most
evolutionists to deny that any major impediment for such change can be ascribed to
large populations. Second, I now realize that my arguments for the channeling of
potential direction and limitation of change apply primarily to levels above
species—to aspects of the developmental Bauplan of anatomical designs that
usually transcend species boundaries, rather than to resistance of populations
against incorporating enough genetic change to yield reproductive isolation from
sister populations.
THE ECOLOGY OF HABITAT TRACKING. This explanation for stasis, long favored
by my colleague Niles Eldredge (1995, 1999), offers a first alternative (in this list)
based on the structuring of species-individuals as ecological entities, rather than on
adaptations or capacities of component organisms—thus taking explanation to a
higher descriptive level of the evolutionary hierarchy. Otherwise, however, habitat
tracking ranks as a conventional Darwinian explanation in calling upon stabilizing
selection to confer stasis upon populations that react to environmental change in
their geographic locale not by evolutionary alteration to new conditions, but rather
by moving with their favored habitat to remain in an unchanged relationship with
their environment of adaptation. Eldredge writes (1999, p. 142): "Paradoxically
(and contrary to at least superficial Darwinian expectations)... stabilizing natural
selection will be the norm even as environmental conditions change—