786 THE STRUCTURE OF EVOLUTIONARY THEORY
and place, therefore ranking only as local populations, rather than true species.)
When well-defined paleospecies have been tested for their correspondence
with modern biospecies, such status has often been persuasively affirmed. Two
recent studies seem particularly convincing. Michaux (1989) studied four living
species of the marine gastropod genus Amalda from New Zealand. Fossils of this
genus date to the upper Eocene of this region, while all four species extend at least
to the Miocene-Pliocene boundary. The four taxa represent good biospecies, based
on absence of hybrids in sympatry, and on extensive electrophoretic study
(Michaux, 1987) showing distinct separation among species and "no detectable
cryptic groupings" (Michaux, 1989, p. 241) within any species. Michaux then used
canonical discriminant analysis to achieve clear morphometric distinction among
the species based on 10 shell measurements for each of 671 live specimens.
He then made the same measurements on 662 fossil specimens from three of
the species (the fourth did not yield enough shells for adequate characterization).
Mean values, in multivariate expression based on all 10 variables, fluctuated
mildly through time (see Fig. 9-8), but never departed from the range of variation
within extant populations—an excellent demonstration of stasis as dynamic
maintenance within well-defined biospecies through several million years.
Michaux concluded (1989, pp. 246-248): "Fossil members of three biologically
distinct species fall within the range of variation that is exhibited by extant
members of these species. The phenotypic trajectory of each species is shown to
oscillate around the modern mean through the time period under consideration.
This pattern demonstrates oscillatory change in phenotype within prescribed limits,
that is, phenotypic stasis."
Jackson and Cheetham's (1990, 1994) extensive studies of cheilostome
bryozoan species provide even more gratifying affirmation, especially since these
"simple" sessile and colonial forms potentially express all the attributes of
extensive ecophenotypic variation (especially in molding of colonies to substrates,
and in effects of crowding) and morphological simplicity (lack of enough complex
skeletal characters for good definition of taxa) generally regarded as rendering the
identification of biospecies hazardous, if not effectively impossible, in fossils.
Moreover, Cheetham had begun his paleontological studies (see discussion on pp.
867 - 870) under the assumption that careful work would reveal predominant
gradualism and refute the "new" hypothesis of punctuated equilibrium—so the
conclusions eventually reached were not favored by any a priori preference!
In a first study—devoted to determining whether biospecies could be
recognized from skeletal characters (of the sort used to define fossil taxa) in
several species within three genera of extant Caribbean cheilostomes—Jackson and
Cheetham (1990) examined heritability for skeletal characters in seven species. In
a "common garden" experiment (under effectively identical conditions at a single
experimental site), they grew F 1 and F 2 generations from embryos derived from
known maternal colonies collected in disparate environments