Punctuated Equilibrium and the Validation of Macroevolutionary Theory 945
Blackburn supplements this empirical evidence for a punctuational divide
with both structural and functional rationales for the inviability of putative
intermediary stages. Viviparity without placentation may be structurally
unattainable because live birth requires that the eggshell become sufficiently thin
"to permit gas exchange in the hypoxic uterine environment" (p. 211)— while such
reduction may entail gas and water exchange (that is, incipient placentation) as a
virtually automatic consequence. Thus, Blackburn argues (p. 210), "placentation is
best viewed as a necessary correlate of viviparity, not as a 'reproductive strategy'
per se."
Intermediacy may be equally unlikely in functional and adaptationist terms as
well. Both endpoints entail costs as well as putative benefits—oviparity in dangers
and energetic requirements of nesting behavior, and in maternal loss of calcium in
making eggshells (p. 211); viviparity in decreased maternal mobility, fecundity, or
clutch size. A hypothetical intermediate that incurs both sets of costs—for
example, the calcium drain from internal shells of hypothetical stage one (still too
thick for placentation), combined with a heightened susceptibility to predators
caused by compromised mobility—without winning greater compensation in
combined benefits, could not compete against either end member of the supposed
trend, and probably would not survive even if patterns of development permitted
evolutionary access to this putatively transitional design.
Finally, in geology, the recent origin of most viviparous lines strongly
supports a punctuational inference. A few origins of squamate viviparity may date
to late Mesozoic or early Cenozoic times (p. 207), but most represent Pliocene or
Pleistocene events. Moreover, taxonomic distribution fully supports the rapidity of
full transition to placentotrophy. More than 60 percent of origins for viviparity
"have occurred at subgeneric levels, and virtually all have arisen at subfamilial
levels." Several origins can be traced to populations of a single species (with other
populations remaining oviparous)—for example, a Pleistocene event within
Lacerta vivipara, and an origin within the past 11 to 25 thousand years within the
Sceloporus aeneus complex (p. 207). The extent of structural differences between
oviparous and viviparous populations of these minimally distant forms (both
temporally and phylogenetically) fully matches the phenotypic separation noted for
the same features in cladistically distant lineages.
- As an indication that data of natural history can provide combined criteria
to permit fine and testable distinctions, Blackburn has been able to reject saltation
and defend punctuated equilibrium as the probable cause and temporal basis of this
well-documented punctuational pattern. Blackburn notes that "under the
punctuated equilibrium model, typical oviparity and viviparity could represent
regions of stasis, with prolonged oviparous egg-retention being a transitory,
intermediate stage between them" (p. 206). The structurally well integrated and
functionally well adapted end members "would contrast with the instability of the
evolutionary intermediate, and prolonged oviparous egg-retention would be a
relatively short-lived (and hence scarce) pattern" (p. 206). By contrast, of course,
saltational models predict the structural