Blackburn, D.G. (1995).
Saltationist and punctuated equilibrium models for the evolution of
viviparity and placentation. Journal of
Theoretical Biology 174: 199-216.
Abstract:
Vertebrate viviparity (live-bearing reproduction), placentation, and
placentotrophy are widely assumed to have evolved as three successive,
gradualistic transformations. From
empirical data and predictive tests on lizards and snakes, this paper indicates
that placentae and a degree of placentotrophy have evolved repeatedly as
necessary correlates of viviparity, not as subsequent modifications. In addition, information derived from
studies of anatomy, physiology, biogeography, and systematics is used to
evaluate new saltationist and punctuated equilibrium models for the evolution
of viviparity. Phylogenetic
reconstruction reveals that more than 100 squamate clades have made the
transition to viviparity and placentation.
However, various phenotypic intermediates postulated by the gradualistic
model are either scarce or unrepresented among known forms, including those in
which viviparity has evolved at specific and subspecific levels.
Evolution in squamates seems to have produced a dichotomy between
two evolutionarily stable patterns: 1) retention of weakly-shelled or
shell-free eggs to term (viviparity), with development of fully functional
placentae; and 2) deposition of shelled eggs at or near the limb bud stage of
development (typical oviparity).
Conflicting functional demands placed on eggshell morphology may
constrain establishment of prolonged, oviparous egg-retention as a viable,
historically stable pattern.
Alternatively, the costs of prolonged egg-retention associated with
decreased female mobility or decreased fecundity may exceed the benefits in
oviparous forms.
Reprints: daniel.blackburn@mail.trincoll.edu