Punctuated Equilibrium and the Validation of Macroevolutionary Theory 905
speciational perspective, successful genera with substantially increasing numbers
of species through time will probably expand their range at both extremes of size,
thus undergoing a speciational trend in variation, not an anagenetic march to larger
sizes!
PARTICULAR CASES. This lowest macroevolutionary level of individual
monophyletic clades has defined the soul of paleontological discourse through the
centuries. Only the histories of particular groups can capture the details that all
vivid story telling requires; the "why" of horses and humans certainly elicits more
passion than the explanation (or denial) of Cope's Law, or the pattern of increasing
mean species longevity in marine invertebrates through time. Yet, even here on
such familiar ground, our explanations remain so near and yet so far—for these
"closer" stories of particular histories must also be reexamined in a speciational
light. Consider just two "classics" and their potential revisions.
- HORSES AS THE EXEMPLAR OF "LIFE'S LITTLE JOKE." As noted
above (Fig. 7-3, and 580-581), the line of horses, proceeding via three major trends
of size, toes and teeth from dog-sized, many-toed, "eohippus" with low-crowned
molars to one-toed Equus with high-crowned molars (see Fig. 9-33 for W. D.
Matthew's classic icon, linearly ordered by stratigraphy) still marches through our
textbooks and museums as the standard-bearer for adaptive trending towards
bigger and better.
I do not deny that, even in a refomulated speciational context, several aspects
of the traditional story continue to hold. MacFadden (1986), for example, has
documented a clear cladal bias towards the punctuational origin of new species at
larger sizes than their immediate ancestors, so both the iconic transition from
ladders to bushes, and the recognition that several speciational events lead to
smaller sizes, even to dwarfed species, throughout the range of the lineage, does
not threaten (but rather reinterprets in interesting ways) the conventional
conclusion that horses have generally increased in size through the Cenozoic.
Moreover, I do not doubt the usual adaptational scenario that a transition from
browsing in soft-turfed woodlands to grazing on newly-evolved grassy plains
(grasses did not evolve until mid-Tertiary times) largely explains the adaptive
context for both the general reduction in numbers of toes from splayed feet on soft
ground to hoofs on harder substrates, and the increasing height of cheek teeth to
prevent premature wear from eating grasses of high silica content.
Nonetheless, a speciational reformulation in terms of changing diversity as
well as anatomical trending tells a strikingly different, and mostly opposite, story
for the clade as a whole. Modern perissodactyls represent but a shade of their
former glory. This clade once dominated the guild of large-bodied mammalian
herbivores, with speciose and successful groups, especially the titanotheres, that
soon became extinct, and with diversity in existing groups far exceeding modern
levels. (The rhinoceros clade once included agile running forms, the hyrachiids,
wallowing hippo-like species, and the indricotheres, the largest land mammals that
ever lived.) Modern perissodactyls exist as three small clades of threatened
species: horses, rhinos, and tapirs.
Horses have declined precipitously from their maximal mid-Tertiary abundance