866 THE STRUCTURE OF EVOLUTIONARY THEORY
majority of Hamilton lineages display virtual stasis from oldest to youngest
samples. Slight nondirectional change is observed in some cases. Such variation
seemingly records very minor evolutionary fluctuation... However, it clearly does
not lead to development of major new grades of morphological development...
Several of the species appear abruptly in the Appalachian Basin near the beginning
of the Hamilton fauna or become locally extinct at its end."
Relative frequencies for entire clades
We add another component to studies of relative frequencies when, in addition to
the thoroughness provided by assessing all lineages within a given time or region,
we add the phylogenetic component of complete coverage for clades (preferably
monophyletic of course, but sometimes paraphyletic in the existing literature).
Obviously, we feel most secure about such phylogenetic assertions when truly
cladistic, or at least stratophenetic, criteria have been used for definitions, but
many studies in this mode employ a standard that, albeit and admittedly less
preferable, probably provides as much confidence in practical utility:
investigations of distinctive taxa known on reliable bio-geographic grounds to be
restricted to a region exhaustively studied. Clades confined to isolated islands,
lakes, or other such distinct and coherent places and environments constitute our
best cases under this criterion.
I have discussed nearly all the best examples in this mode under other
headings of this chapter, and will only make brief reference here. Several "classic"
mammalian lineages fall into this category of excellent cladistic definition and
overwhelming domination by the punctuational pattern of stasis within species and
geologically abrupt transitions between—all despite (or rather, in a punctuational
reformulation, because of!) such celebrated evidence for sustained and important
trends. I include here the excellent evidence for horses (see p. 905) and the spottier
but still persuasive data on hominid evolution (see pp. 908-916)—in each case, for
clades well delimited both by morphology and geography.
Among such geographically confined clades, Vrba's classic studies (1984a
and b) of African antelopes stand out for detailed data on one of the most
successful and speciose of vertebrate higher taxa. In the maximally diverse tribe
Alcelaphini (including blesbucks, hartebeests, and wildebeests), the Quaternary
record includes 25 species, all with a geologically sudden origin in recorded data,
and with cladogenesis as a reliably inferred mode of origin for at least 18 species.
Several species lived for 2 million years or longer in stasis, and no ancestors with
incrementally transitional morphologies have been found for any of these forms.
I continue to be amazed by the skewed interpretation often imposed by
gradualistic expectations upon data for clades that seem, at least in my partisan
judgement, clearly dominated by punctuated equilibrium in overall relative
frequency. For example, in a well-known work, White and Harris (1977) used the
Plio-Pleistocene record of African pigs for supposed validation of gradualism as a
primary guide in biostratigraphic resolution (particularly of