882 THE STRUCTURE OF EVOLUTIONARY THEORY
whole, not more. Lieberman and Dudgeon write (p. 231): "If stabilizing selection
played a prominent role in maintaining stasis one would expect to find relatively
little morphological change through time within a single environment." Williams
(1992) has made a similar argument, at a lower scale, against stabilizing selection
by emphasizing that the copiously, and lovingly, documented efficacy of natural
selection in short-term situations of human observation—from beaks of Darwin's
finches to industrial melanism in Bistort betularia—makes stabilizing selection
doubtful as a general explanation for such a pervasive phenomenon as stasis within
paleospecies.
But when we consider this finding in supraorganismal terms, with demes as
Darwinian individuals, an evident and sensible interpretation immediately
emerges. A temporally coherent population may adapt gradually and continually
while tracking one of several paleoenvironments inhabited by a species. But how
can these anagenetic changes spread adaptively through an entire species
composed of several other subpopulations, each adapted to (and tracking) its own
paleoenvironment through time? No single morphology can represent a functional
optimum for all habitats. In this common, and probably canonical, situation for
species in nature, stability emerges as a form of "compromise" in most
circumstances, a norm among "competing" minor changes that are, themselves,
probably distributed more or less at random around a standard configuration, with
each particular solution generally incapable, in any case, of spreading through all
other demes of the species in the face of better locally adaptive configurations in
most of these demes.
Of course, one can think of several obvious alternative structures where
gradual change might be noted—lack of metapopulational division, with the entire
species acting as a single deme, or some accessible and general biomechanical
advantage that might be adaptive in all demes. But such circumstances may be
uncommon—however important by cumulation in the overall history of life—in
any general sample of species within a clade at any given time, thus accounting for
the predominant relative frequency of stasis among all species, and for the relative
rarity of anagenetic change within species as well.
Lieberman and Dudgeon summarize their proposed explanation by writing
(1996, p. 231)
Stasis may emerge from the way in which species are organized into
reproductive groups occurring in separate environments.... The morphology
of organisms within each of these demes may change through time due to
local adaptation or drift, but the net sum of these independent changes will
often cancel out, leading to overall net stasis... Only if all morphological
changes across all environments were in the same direction in
morphospace, or if morphological changes in a few environments were
very dramatic and in the same direction, would there be significant net
change in species morphology over time... Thus, as long as a species
occurs in several different environments one would predict on average it
should be resistant to change.