Punctuated Equilibrium and the Validation of Macroevolutionary Theory 749
selective pressures for change: "You speak of these animals as having being
exposed to a vast range of climatal changes from before to after the Glacial period.
I should have thought, from analogy of sea-shells, that by migration (or local
extinction when migration is not possible) these animals might and would have
kept under nearly the same climate."
Searching for another way to explain the absence of anticipated (and gradual)
change, Darwin then argued that altering climates may generally imply
evolutionary modification, but that groups in serious decline, including elephants,
often become stalled in their capacity to vary, and especially to form new taxa: "A
rather more important consideration, as it seems to me, is that the whole
proboscidean group may, I presume, be looked at as verging towards extinction...
Numerous considerations and facts have led me in the Origin to conclude that it is
the flourishing or dominant members of each order which generally give rise to
new races, sub-species, and species; and under this point of view I am not at all
surprised at the constancy of your species." But if Darwin had not been surprised,
or at least disturbed, why did he try so hard to reconcile this unexpected
phenomenon with his general theory? Falconer, in any case, replied that elephants
remained in vigor, and could not be considered as a group on the verge of
elimination.
I recount this story at some length, as an introduction to punctuated
equilibrium, both because Falconer and Darwin presage in such a striking manner,
the main positions of supporters and opponents (respectively) of punctuated
equilibrium in our generation, and because the tale itself illustrates the central fact
of the fossil record so well—geologically abrupt origin and subsequent extended
stasis of most species. Falconer, especially, illustrates the transition from too easy a
false resolution under creationist premises, to recognizing a puzzle (and proposing
some interesting solutions) within the new world of evolutionary explanation. Most
importantly, this tale exemplifies what may be called the cardinal and dominant
fact of the fossil record, something that professional paleontologists learned as
soon as they developed tools for an adequate stratigraphic tracing of fossils
through time: the great majority of species appear with geological abruptness in the
fossil record and then persist in stasis until their extinction. Anatomy may fluctuate
through time, but the last remnants of a species usually look pretty much like the
first representatives. In proposing punctuated equilibrium, Eldredge and I did not
discover, or even rediscover, this fundamental fact of the fossil record.
Paleontologists have always recognized the longterm stability of most species, but
we had become more than a bit ashamed by this strong and literal signal, for the
dominant theory of our scientific culture told us to look for the opposite result of
gradualism as the primary empirical expression of every biologist's favorite
subject—evolution itself.
Testimonials to Common Knowledge
The common knowledge of a profession often goes unrecorded in technical
literature for two reasons: one need not preach commonplaces to the initiated; and
one should not attempt to inform the uninitiated in publications