Punctuated Equilibrium and the Validation of Macroevolutionary Theory 797
We did not urge this correlation between speciation events and morphological
change in a self-serving and circular manner—i.e., only because the pattern of
punctuated equilibrium could be best defended thereby. We did, of course,
recognize the logical link, as in the following statement from Gould, 1982c, p. 87
(see also Gould and Eldredge, 1977, p. 137): "Reproductive isolation and the
morphological gaps that define species for paleontologists are not equivalent.
Punctuated equilibrium requires either that most morphological change arise in
coincidence with speciation itself, or that the morphological adaptations made
possible by reproductive isolation arise rapidly thereafter." But we based our
defense of this proposition upon a large, and then quite standard, literature
advocating a strong negative correlation between capacity for rapid evolutionary
change and population size. Small populations, under these models, maintained
maximal prospects for rapid transformation based on several factors, including
potentially rapid fixation of favorable variants, and enhancement of differences
from ancestral populations by interaction of intense selection with stochastic
reasons for change (particularly the founder effect) that can only occur with such
effective speed in small populations. Large and stable populations, by the converse
of these arguments, should be sluggish and resistant to change.
This literature culminated in Mayr's spirited defense for "genetic revolution"
as a common component of speciation (first proposed in a famous 1954 article, and
then defended in extenso in the 1963 book that served as the closest analog to a
"bible" for graduate students of my generation). Since Mayr (who coined the name
"founder effect" in this context) also linked his concept of "genetic revolution" to
the small, peripherally isolated populations that served as "incipient species" in his
influential theory of peripatric speciation—and since we had invoked this theory in
our original formulation of punctuated equilibrium (Eldredge and Gould, 1972)—
our defense of a link between speciation and concentrated episodes of genetic (and
phenotypic) change flowed logically from the evolutionary views we had
embraced. Thus, we correlated punctuations with the extensive changes that often
occurred during events of speciation in small, peripherally isolated populations;
and we linked stasis with the expected stability of large and successful populations
following their more volatile and punctuational origins as small isolates.
I can claim no expertise in this aspect of neontological evolutionary theory,
but I certainly acknowledge, and must therefore provisionally accept, the revised
consensus of the past twenty years that has challenged this body of thought, and
rejected any general rationale for equating the bulk of evolutionary change with
events of speciation in small populations, or with small populations in any sense.
As I read the current literature, most evolutionists now view large populations as
equally prone to evolutionary transformation, and also find no reason to equate
times of speciation—the attainment of reproductive isolation—with acceleration in
general rates of genetic or phenotypic change (see, for example, Ridley, 1993; and
Williams, 1992). (I do, however, continue to wonder whether the Mayrian
viewpoint might still hold