852 THE STRUCTURE OF EVOLUTIONARY THEORY
from direct paleontological records, but as firm inferences based on species flocks
in lakes or on islands of known and recent origin (with African cichlid fishes as the
classic case of modern evolutionary biology). These evolutionary "explosions"
often produce several hundred species in just a few thousand years, and must be
ranked as punctuational with a luxurious vengeance! But such circumstances do
not represent a norm for most speciation in most clades, and such an unusual
phenomenon, however stunning and however well documented, cannot suffice to
validate a proposed generality.
The punctuational origin of many species can be accurately timed with direct
paleontological data. Lister (1996) calculated a maximum of only 5000 years for
the Quaternary evolution of dwarfed woolly mammoths on Wrangel Island, and
6000 years for the dwarfed red deer of Jersey. The punctuational origin of the
marginellid gastropod Prunum christineladdae, based on the study of Nehm and
Geary (1994), took 73,000 to 275,000 years, and spanned 0.6 to 2.5 percent of the
full duration of the ancestral species. Reyment (1982) calculated outside limits of
100,000 to 200,000 years (perhaps a good deal less) for origin of the Cretaceous
ostracode Oertiella chouberti from its ancestor, O. tarfayaensis.
GEOGRAPHY. I have already discussed this important tool for validating
punctuated equilibrium by gathering data at a more inclusive and finer scale than
the local documentation of a literal punctuation. On pp. 840-845, I described cases
where geographic data affirmed an allopatric and punctuational event of
cladogenesis, thus demonstrating that the abrupt appearance of a descendant
species truly represents punctuated equilibrium, and not just a migrational
incursion of a species that originated by an uncertain mode in an unknown place
(Wei and Kennett, 1988, for protistans; Williamson, 1981, for mollusks; Lister,
1996, and Heaton, 1993, for mammals, among many others. Albanesi and Barnes
(2000) present a particularly well documented case both for allopatric and
punctuational origin of new taxa and survival of ancestors in their original regions
for a lineage of Ordovician conodonts).
MORPHOMETRIC MODE. By quantitative study of patterns in morphological
transition between ancestral and descendant species, several criteria of inference
can increase our confidence in the identification of punctuated equilibrium, both by
establishing a case for direct filiation rather than simple replacement by a taxon
evolved elsewhere, and by indicating a punctuational mode for the cladogenetic
event. As illustrations of this approach, consider three effective morphometric
arguments:
- Visually extensive change (supposedly requiring many independent inputs
expressed over substantial time) can arise as coordinated consequences of one, or
few, generating factors, and can therefore readily be accomplished at a
punctuational tempo. This "standard" argument has a long pedigree, and serves
many purposes, in evolutionary theory (see Chapter 10, for example, on "positive"
constraints). In the context of punctuated equilibrium,