Punctuated Equilibrium and the Validation of Macroevolutionary Theory 847
and two of Miohippus, all contemporaries). Prothero and Shubin conclude: "This is
contrary to the widely-held myth about horse species as gradualistically-varying
parts of a continuum, with no real distinctions between species. Throughout the
history of horses, the species are well-marked and static over millions of years. At
high resolution, the gradualistic picture of horse evolution becomes a complex
bush of overlapping, closely related species."
To end this section with a particularly instructive example, punctuated
equilibrium has frequently been saddled with the charge that inherent limitations of
paleontological data yield biased results, artificially and superficially favorable to
the theory—with Darwin's classic argument against a literal reading of
punctuations as the conventional antidote. However, an opposite bias may also be
significant, and may lead to serious underestimation of punctuated equilibrium in a
circumstance likely to be quite common: when a descendant, fully distinct at its
origin but initially rare, enters the ancestral area, and then increases steadily in
relative abundance as the ancestor declines to extinction. The true evolutionary
pattern will be fully punctuational, with stasis in both ancestor and descendant
throughout, and with abrupt geological origin of the descendant. But if we misread
the event as a tale of anagenetic transformation, and if the two species overlap
extensively in ranges of variation, then we will misinterpret the full pattern as
transformation by anagenesis, rather than replacement with steadily increasing
relative abundance of the descendant species.
The important distinction between these interpretations can be made with
appropriate statistical tools applied to samples of sufficient size—but the
punctuational alternative must be conceptually available to suggest such a test. In
this subtle sense, among so many other more overt reasons explored in this book,
expectations of gradualism seriously restrict our range of potential explanations for
evolutionary modes and tempos—and punctuated equilibrium therefore becomes
both suggestive and expansive, whether or not the hypothesis holds in any
particular case.
In an elegant demonstration of this principle, Heaton (1993 and 1996 for data
in extenso) showed that a classic case of supposedly gradualistic anagenesis in
Oligocene rodents from the western United States really represents a case of
replacement. Heaton writes (1993, p. 297): "Statistical investigation of large
samples suggests instead that two closely related species coexisted, and the shift in
mean size that was thought to represent anagenesis actually represents
replacement."
Heaton demonstrated the distinct character of the two taxa both by bimodality
in their joint occurrences (Fig. 9-20), and by showing that the two species
maintained distinctly different geographic ranges (with overlap in Nebraska and
eastern Wyoming, but only the descendant taxon living at the same time in the
Dakotas—see Figs. 9-21 and 9-22). The small species, Ischyromys parvidens,
predominates in the early Orellan, although the larger I. typus already occurs low
frequency in the same strata. I. typus then increases,