942 THE STRUCTURE OF EVOLUTIONARY THEORY
than traditional views would allow between successive phases of ontogeny: "Long
periods of little net change, with functionally minor modifications in
developmental mechanisms and larvae, seem to be the normal mode of evolution.
This near stasis is interrupted on occasion by rapid, extensive, and mechanically
significant changes that coincide with switches in life history strategy ... Rapid
modifications can arise in developmental mechanisms that have been conserved for
hundreds of millions of years."
Note the striking similarity of language (with analogs of stasis and
punctuation)—and of evolutionary style in tempo and mode—between punctuated
equilibrium and Wray's description of phenotypic and ecological shift at these
much larger scales of morphological change and temporal extent (with the analog
of stasis persisting for hundreds of millions of years). These similarities in style
and import seem to mark a genuine conceptual "homology" based on similar
structural principles regulating the nature of change in complex systems.
As a final example of the fruitfulness and detailed testability of punctuational
models for the origin of morphogenetic novelty, Blackburn's (1995) remarkable
study of "saltationist and punctuated equilibrium models for the evolution of
viviparity and placentation" deserves special notice for the author's clarity in
specification of hypotheses, and for his richness and rigor in attendant
documentation. Blackburn treats the multiple evolution of viviparity in squamate
reptiles (lizards and snakes)—a much better case for studying this phenomenon
than the group that most of us emphasize for parochial reasons (the Mammalia),
for extant squamate species include many examples in all stages of the process.
This taxonomic richness permits clear distinction of gradualistic vs. punctuational
alternatives, and also provides sufficient data to distinguish between punctuated
equilibrium and saltation as the dominant punctuational style. Moreover, and
largely because the subject has been embraced as a workable surrogate for
unresolvable questions about mammalian evolution, the origin of viviparity in
squamate reptiles has become a classical case, and has therefore generated an
extensive literature to illustrate (however unintentionally) some major biases of
evolutionary argumentation. The power of Blackburn's study resides in three
interrelated themes:
- Gradualistic scenarios have dominated the classical literature in ways that
authors rarely defend, or even recognize. In particular, previous workers have
assumed that three apparent stages in the "perfection" of live bearing must
represent an actual historical sequence gradually and incrementally evolved by all
lineages that reach the "last stage"—viviparity or live birth, placentation for gas
exchange and water intake, and placentotrophy for embryonic nutrition. Blackburn
writes (1995, pp. 199-201): "Viviparity and placentation in squamates have stood
for over half of a century as examples of gradual evolution ... Even recent
reviewers have not considered the applicability of alternative evolutionary
models."
The supposed evidence for such gradualism consists largely of inferences
drawn from structural series of extant forms, without affirmation, or even
consideration, of an explicit phylogenetic hypothesis that the successive