740 THE STRUCTURE OF EVOLUTIONARY THEORY
while features that enhance longevity also suppress the rate of speciation. For
example, small populations in stressful environments are especially prone to both
speciating and dying; while large, global populations of marked stability and great
mobility (like Homo sapiens and Rattus rattus) are remarkably resistant to
extinction (unless, like one of the above, they evolve an odd capacity for potential
self-destruction), but ill-equipped to form the isolated populations that can generate
new species.
For a third limiting constraint of brakes on the amount of available variation
for selectional processes (line IVB3), infrequency of new mutation may play an
important role at the organismal level (not so often in sexual forms, where
recombination greatly boosts the amount of variability among individuals, but
usually a defining limit in asexuals, and perhaps the major reason for the rarity and
marginal status of asexuals among complex Metazoa, but not in unicells with short
generations). At the species level, variation per individual may be more than
adequate (given the forced correlation of birth with change), but many clades
contain too few individuals, giving birth too rarely, for very efficient selection
(Fisher's argument—see page 645).
For a final factor among limiting constraints (in this abbreviated list), brakes
on development act strongly at both levels (line IVB4). Ever since the inception of
modern embryology, von Baer's (1828) laws have defined the hold placed by
ontogenetic intricacy upon potentials for change in complex Metazoa. At the
species level, the hold of homology (as expressed in all the factors, genetic and
otherwise, that limit the amount of change per speciation event) functions as a
developmental constraint in the same basic manner— that is, by limiting the
difference that can separate a parent and its immediate offspring.
All these sources of limitation also contribute to the more important positive
aspects of constraint, as channeling or enhancing preferred directions for change.
In the category of positive channeling by structure (line IVB5), ontogenetic
pathways already established in the lives of organisms provide by extension, or by
relative shuffling of rates among components (see Gould, 1977b), the classic mode
of constrained and substantial change in organismic evolution—thus explaining the
importance of heterochrony as a morphogenetic phenomenon (Jones and Gould,
1999; McNamara and McKinney, 1991). At the upper level of speciational trends
within clades, structural rules and differential ease of modifiability among parts
and correlations of Bauplan play the same role of directing and accelerating
change along certain preferred pathways. Liem (1973), for example, showed how a
set of small and accessible changes in a jaw muscle, the fourth levator externi,
could greatly alter the adaptive feeding devices of cichlid fishes (but not of other
related groups), thus helping to explain the rapidly evolved species flocks of this
clade in several African lakes.
In a second category of positive channeling by directed variability from levels
below, the organismic level experiences no important effect because such drives
will generally be suppressed by organismic selection. But the driving