Punctuated Equilibrium and the Validation of Macroevolutionary Theory 853
this theme establishes plausibility for temporal compression of visually substantial
change into a single cladogenetic event at a punctuational tempo. The argument
also proceeds in several modes, including inductive approaches based on
"covariance sets" (Gould, 1984b) of correlated characters transformed together
along a single multivariate axis. For example, the transition documented by
Goodfriend and Gould (1996) involved several measures coordinated by a single
change in direction and rate of growth along the axis of coiling—a coherently
correlated pattern running orthogonal to, and therefore independent of, the standard
covariance set representing shell size alone. Since the cladogenetic event altered
shape (as expressed along the axis of coiling), but not size, this multivariate
separation established the source of morphometric change and also revealed its
unitary nature in modified growth.
Among examples of the opposite deductive approach, based on fitting an
apparent complexity of observed changes to a simple model of underlying
generation, Smith and Paul (1985) recognized a suite of alterations through a
punctuational event in Cretaceous echinoids as coordinated consequences of a
single heterochronic change; and Benson (1983) explained a "punctuational event"
(1983, p. 398) in the ostracode Poseidonamicus as a set of secondary, and
mechanically automatic, accommodations of the carapace to a primary change in
shape.
- Patterns of changing variation through a "dissected" punctuation may
reveal a cladogenetic mode of direct filiation. Empirical patterns of variation may
permit distinction between punctuational incursion from elsewhere (a migrational
event of uncertain interpretation) and cladogenesis in situ. To contrast two studies
previously reported, Heaton (1993) found consistent and unaltered bimodality in a
vertical sequence of Oligocene rodents, with an appearance of overall gradualism
arising as an artifact of directionally changing relative abundances. Such a
pattern—while confirming stasis and showing the utility of punctuated equilibrium
as a generator of alternative hypotheses—cannot resolve the descendant's mode of
origin, for the new species enters the fossil record in full and complete distinction.
In Williamson's study of African lake mollusks (1981), however, a distinctive
pattern of variation (see previous discussion on page 769) implicates cladogenesis
in situ—for Williamson's fine-scale vertical resolution allowed him to discern an
initial period of expanded intrapopulational variation followed by a reduction back
to ancestral levels, but now centered about the altered mean of the new species. - Comparison of within- and between-species variation as a test of
extrapolationism. If new species arise by gradualistic anagenesis, then the direction
of selection within populations and the pattern of temporal variation during the life
of a species should mirror the morphological changes between species in a
geological trend. But Shapiro (1978) estimated natural selection in the Miocene
scallop Chesapecten coccynelus by comparing specimens that died as juveniles to
those that survived to adulthood. He measured the direction of implied change as
not only different from, but actually orthogonal to, the distinction between this
species and its descendant, Chesapecten nefrens.