Punctuated Equilibrium and the Validation of Macroevolutionary Theory 943
stages represent a cladistic sequence. (Even worse, a phylogenetic inference has
often been based only upon the series itself—a flagrantly circular argument that
validates the conclusion by the hypothesis supposedly under test.) For example,
extant oviparous species do vary substantially in the stage of development at which
the eggs are laid—and researchers have generally assumed that a linear ordering of
such a series must represent an evolutionary continuum "on the way" to viviparity:
"The inferred continuum of developmental stages at oviposition among squamates
commonly is interpreted as evidence for a gradual increase in the proportion of
development occurring in the female reproductive tract" (ibid., p. 201).
- Blackburn marshalls an impressive array of data from a broad range of
fields—taxonomy, development and geology, in particular—to affirm an
alternative punctuational scenario for the evolution of live birth, with simple
viviparity, placentation and placentotrophy as three distinct modes, not three way
stations in a progressive sequence. (I suspect that our gradualistic biases have been
particularly intrusive in this case because we unconsciously read the squamate
story in a mammalian perspective that makes placentotrophy the "obvious" goal of
any trend to live bearing.)
In taxonomy, viviparity has originated more than 100 times among squamate
reptiles (ibid., p. 202). But cladistic data have provided not a single case of
correspondence between branching order and the four structural stages of the
hypothetical trend: ovipary, vivipary (live birth without placentation of embryos),
placentation, and placentotrophy. Blackburn writes (1995, pp. 201-202): "Clines of
phenotypic variation that can be invoked to support gradualistic evolution of
viviparity and placentotrophy tend to be composites of unrelated species
representing multiple lineages ... Despite the documentation of over 100
evolutionary origins of viviparity in squamates ... available evidence has not yet
permitted construction of a single, complete phenocline of parity modes and
embryonic nutritional patterns out of representatives of a single clade."
In structure and development, Blackburn coordinates several lines of evidence
to argue that intermediary forms between any two stages in the hypothetical trend
either cannot be found, or exist only rarely and in a tenuous state (because such
transitional phenotypes would experience either architectural problems in
construction or adaptive insufficiencies in function). For example, if viviparity
evolved by progressive delay of oviposition, then we might expect, among extant
species, "a full continuum of developmental stages... representing steps in the
parallel evolutionary transformations that have occurred independently (and
perhaps to different degrees) in various lineages" (p. 202). Instead (see Fig. 9-35),
the distribution of developmental stages at oviposition shows marked bimodality,
with species either depositing eggs containing embryos in the pharygula/limb bud
stages (with near normality or minor left skewing for this lower mode, and no right
skew in the direction of the putative trend) or else retaining the eggs to term and
then giving birth to live young.
Moreover, the supposed development of placentation, and then of
placentotrophy,