812 THE STRUCTURE OF EVOLUTIONARY THEORY
on empirical results. Several researchers have noted that punctuated equilibrium
implies a primary prediction about patterns of genetic differences among species: if
most change accumulates at ruptures of stasis during events of speciation, and not
continuously along the anagenetic history of a population, then overall genetic
differences between pairs of species should correlate more closely with the
estimated number of speciation events separating them, than with chronological
time since divergence from common ancestry. (This prediction might be clouded
by several factors, including the foregoing discussion on attributing the bulk of
genomic change to continuity at a lower level, and a number of potential reasons
for discordance between phenotypic effect and extent of responsible genetic
change. But I certainly will not quibble, and I do allow that punctuated equilibrium
suggests the broad generality of such a result.)
In the early days of debate about punctuated equilibrium, Avise (1977)
performed an interesting and widely discussed test. In comparing genetic and
morphological differences among species in two fish clades of apparently equal
age but markedly different frequencies of speciation, Avise found a higher
correlation of distances with age than with frequency of branching, and therefore
favored gradualism over punctuated equilibrium as an explanation of his results.
But Mayden (1986) then showed that Avise's test did not apply well to his chosen
case (primarily because we cannot be sure of roughly equal antiquity for the two
clades). He then argued, as several supporters of cladistic methodology had urged,
that such tests should be applied only to well-confirmed cladistic sister groups—
for, in such cases, even if paleontological data permit no certainty about the actual
time of joint origin from common ancestry, at least we can be confident that the
two clades are equally old! Mindel et al. (1989) then performed such a properly
constituted test on the reptilian genus Sceloporus, and more loosely on allozymic
data in general, and found a positive correlation between evolutionary distance and
frequency of speciation—thus validating the primary prediction of punctuated
equilibrium.
Empirical tests of conformity with models
Limitations of the fossil record restrict prospects for testing punctuated equilibrium
by inductive enumeration of individual species and lineages. Cases with sufficient
resolution may not be common enough to establish a robust relative frequency; or
systematic biases based on imperfections in the fossil record may lead to artifactual
preferences for punctuated equilibrium—thus making the data unusable as a fair
test for a minimal frequency. (I do not regard these problems as particularly
serious, and I will provide several examples of adequate resolution in the next
section of this chapter. But we should, in the light of these difficulties, also be
exploring other ways of testing punctuated equilibrium, as considered below.)
In another strategy that has been pursued by some researchers, but could (and
should) be exploited to a much wider and more varied extent, we might
characterize, in quantitative fashion, broader patterns in the deployment of