Punctuated Equilibrium and the Validation of Macroevolutionary Theory 833
at least two spanning about five million years, and display apparent stasis in most
characters."
Several analogs of Gingerich's classic studies on gradualism have provided
strong evidence for stasis, thus proving diversity of modes, even where gradualism
had been most strongly asserted as an exclusive pattern. Gingerich had studied the
small condylarth Hyopsodus in early Eocene rocks from the Big Horn Basin of
northwestern Wyoming. West (1979), however, found only stasis for the same
genus from slightly younger Middle Eocene rocks from the Bridger Formation of
southwestern Wyoming. West concluded (1979, p. 252): "Bridger Formation
Hyopsodus data seems to show little size change through approximately one
million years. This stasis or equilibrium condition ... is the only well developed
pattern in Bridger Hyopsodus." Schankler (1981) then analyzed another genus, the
condylarth Phenacodus, from the Big Horn Basin strata used by Gingerich to
document gradualism in different taxa, and found only stasis within species (with
abrupt transitions between species—a pattern that Schankler interpreted, correctly
in my view, as a probable result of migration into a local area, rather than
punctuational speciation in situ). He concluded (1981, p. 137): "The long-term
stasis in morphology and size shown by the four species of Phenacodus conforms
to the pattern expected in a model of evolution by punctuated equilibria."
As for the mammal we all love best (see pp. 908-916 for a more complete
analysis), gradualism had long reigned as an unquestioned (and often quite
unconscious) assumption in hominid evolution. An extensive, historically
sanctioned set of dogmata, from ideas about "missing links" to the "single species
hypothesis," presupposed gradualism as a philosophical foundation. An early study
by Cronin et al. (1980)—which would not be defended by several of its coauthors
today—made the classic error of regarding a monotonically changing set of mean
values as virtual proof for anagenetic gradualism. (Such data cannot distinguish the
stair steps of punctuated equilibrium from the same empirical pattern produced by
gradualism in highly incomplete sections.)
The spotty data of hominids offer little opportunity for adequate testing of
such ideas (and we wouldn't even think of applying an apparatus of this kind to
such a poor example if we didn't care so much about the particular case).
Nonetheless, I am gratified by some strong hints of substantial stasis in several
hominid species, especially for increasingly persuasive data on the importance of
apparently punctuational speciation in this small clade during a crucial million year
African interval (ca. 2-3 my B.P.) that featured the putative origin of at least half a
dozen hominid species. Rightmire's early claims (1981,1986) for stasis in Homo
erectus have been strongly challenged (Wolpoff, 1984), though the jury has surely
not yet come in (despite a tentative vote from this juror, despite his general biases
in the other direction, for at least some fairly persuasive gradualism within this
species).
But two apparently sound cases of stasis have attracted substantial attention
while we should also not neglect, if only for its radical meaning in the light of
previous assumptions, the short-term stasis of Homo sapiens, at least from the
earliest Cro-Magnon records in Europe (about 40,000 years B.P. to