Punctuated Equilibrium and the Validation of Macroevolutionary Theory 895
insight. Moreover, this resistance arises for the most ironical of recursive
reasons—namely, that other biases of the Darwinian tradition, particularly the
reductionist and extrapolationist premises discussed throughout this book as the
essential Darwinian tripod, have forestalled an application of Darwin's deepest
insight to nature's grandest scale!
We have conceptualized macroevolution as temporal fluxes of adaptive form,
with either some notion of an average phenotype for a clade (corresponding to
vernacular ideas of the "ordinary" or "normal"), or some extreme value
(representing our view of the "promise" or "potential" of a collectivity) standing as
a surrogate or summary for all variation, in both form and number of species,
within the clade under consideration. This attitude has led us to embrace a set of
patent absurdities that rank, nonetheless, as received wisdom about the history of
life, and that we continue to support in a passive way because the fallacies can only
become apparent when we reconceive macroevolution as an inquiry centered upon
the changing sizes and variabilities of clades based on the fates of their component
Darwinian individuals—their species, construed as "atoms" of macroevolution (at
least for sexually reproducing organisms).
Thus, we usually summarize the history of life as a drama about generally
increasing complexity of form (with bacteria, beetles and fishes left successively
behind, despite their evident prosperity) when, at the very most, such a theme can
only apply to a small group of species on the right tail of life's general
distribution—and when, by any fair criterion generally employed by evolutionists,
bacteria have always dominated the history of life from their origins in exclusivity
more than 3.5 billion years ago to their current mastery of a much more diverse
world. And we have chosen horses as our textbook and museum hall example of
progressive evolution triumphant, when modern equids represent a pitiable
remnant of past diversity, a small clade entirely extinguished in its original, and
formerly speciose, New World home, and now surviving as only a half dozen or so
species of horses, asses and zebras at the more hospitable termini of past
migrations. Horses, moreover, represent failures within a failure—for the once
proud order of Perissodactyla now persists as only three small clades of threatened
species (tapirs, rhinoceroses and equids, with the last receiving an artificial boost
for human purposes), while the once minor order of Artiodactyla now dominates
the guild of large, hoofed herbivorous mammals as one of evolution's great success
stories. But we will not grasp these evident patterns, truly generated by changing
diversity of species, if we continue to dwell in a conceptual prison that frames the
history of life as a flux from monad to man, and the phylogeny of horses as a
stately race from little splay-footed eohippus to the one-toed nobility of Man O'
War.
The key to a more expansive formulation—also a more accurate depiction in
the language of probable causes based on genuine evolutionary agents— lies in
recasting this discourse about fluxes of means or extreme values within clades as a
history of the differential origins and fates of species, as organized by nature's
genealogical system (that is, evolution itself) into the monophyletic