Punctuated Equilibrium and the Validation of Macroevolutionary Theory 907
to modern marginality in both place and number. As O.C. Marsh proved to T.H.
Huxley in a famous incident in the history of paleontology (see Gould, 1996a, pp.
57 - 58), horses evolved in America only to peter out and disappear completely in
their native land, surviving only as a few lines of Old World migrants. In North
America alone, from 8 to 15 million years ago, an average of 16 species lived
contemporaneously. As recently as 5 million years ago, at least six sympatric
species of horses lived in Florida alone (MacFadden et al., 1999). Only 6-8 species
of horses inhabit the entire earth today.
Moreover, if one wished to argue that a particular affection for Equus permits
a restricted focus on this pathway through the bush alone, with the pruning of all
other pathways regarded as irrelevant; and if one then claimed that this sole
surviving pathway illustrates the predictable excellence of increasing adaptation by
expressing a pervasive cladal trend (so that any surviving lineage would still tell
the same adaptive tale for its own particular sequence); then the speciational view
must debunk this last potential version of the old triumphalist attitude as well. If all
paths through the equid bush led to the same Rome of modern Equus, or even if all
major and prosperous paths—as measured by species range, diversity, or any
conventional attribute of phyletic success—moved in the same general direction,
then one might separate issues of species numbers, where the decline of equids
through time cannot be denied, from the question of cladal direction, where the
classic trend could still be asserted as predictable and progressive, even while the
clade declined in diversity.
But reasonable, and very different, alternative scenarios for cladal direction
remained viable until the recent restriction of the clade to the single genus Equus.
MacFadden's (1988) study of all identifiable ancestral-descendant pairs showed 5
of 24, or more than 20 percent, leading towards decreased size. The general bias
remains clear, but alternative scenarios also remain numerous and entirely
reasonable (albeit, obviously, unrealized in the unpredictable history of life).
For example, the dwarfed genus Nannippus lived as a highly successful equid
subclade for eight million years (far longer than Equus so far) at substantial species
diversity (at least four named taxa) in North America, becoming extinct only 2
million years ago. Suppose that, in the reduction of the clade to one genus,
Nannippus had survived and the forebears of Equus died? What, with this small
and plausible alteration by "counterfactual history," would we then make of the
vaunted and canonical trend in the evolution of horses? Nannippus did not exceed
ancestral eohippus in size, and still grew three toes on each foot. Nannippus molars
were as relatively high-crowned as those of modern Equus, but I doubt that any
biologist would challenge my noncynical scenario based on psychological salience:
If Equus had died, and Nannippus survived as the only genus of modern horses,
this "insignificant" clade would now be passing beneath our pedagogical notice as
just one more "unsuccessful" mammalian line—like aardvarks, pangolins, hyraxes,
dugongs, and several others—equally reduced to a shadow of former success.