846 THE STRUCTURE OF EVOLUTIONARY THEORY
a weight that has only increased in the light of discoveries since then (see Leakey
et al., 2001), particularly for a vigorous phase of cladogenesis 2-3 million years
ago, leading to at least half a dozen hominid species (see Johan-son and Edgar,
1996).
On the same subject of punctuational and cladogenetic reformulations for
classic evolutionary trends previously framed (and widely celebrated in both
textbook and story) as exemplars of anagenetic gradualism, the phylogeny of
horses has been rewritten as a copious cladogenetic bush replete with ancestral
survival in the very parts of the sequence once most firmly read as a tale of linear
progress. For example, Prothero and Shubin (1989) have shown that the Oligocene
transition from Mesohippus to Miohippus conforms to punctuated equilibrium,
with stasis in all species of both lines, transition by rapid branching rather than
phyletic transformation, and stratigraphic overlap of both genera (one set of beds in
Wyoming has yielded three species of Mesohippus
9 - 19. From McHenry (1994). The hominid record is spotty, but the basic pattern of substantial
stasis within several species—particularly A. afarensis—and numerous branching points with
persistence of putative ancestors lends support to the model of punctuated equilibrium.