816 THE STRUCTURE OF EVOLUTIONARY THEORY
"of course the magnitude of change in both form and diversity correlates with
number of branching events (what else could a 'radiation' mean). But adaptive
radiations only accelerate the frequency of branching in response to ecological
opportunity ('open' environments just invaded or just cleared out by extinction);
they do not affect the modality of change. I will allow that, in adaptive radiations,
most new species arise in less time than usual, but still gradualistically. If full
speciation takes half the average time (one million rather than a modal two million
years, for example), but still occurs imperceptibly and still occupies a large
percentage of an average species's lifetime, then gradualism encounters no threat in
adaptive radiation."
However, in another crucial sense, at least one of Stanley's tests does illustrate
the most salutary potential role for punctuated equilibrium: its capacity to act as a
prod for expansive thought and new hypotheses, whatever the outcome of the
empirical debate about relative frequency. Paleontologists had been truly stymied
in their thinking about the important and contentious topic of "living fossils."
Neither of the two conventional explanations could claim any real plausibility.
Every textbook that I ever consulted as a student dutifully repeated the old saw that
living fossils had probably achieved optimal adaptation to their environment.
Therefore, no alternative construction could selectively replace an ideal form
achieved so long ago. But no one ever presented any even vaguely plausible
evidence for such a confident assertion. Why should horseshoe crabs lie closer to
optimality than any other arthropod? What works so well in the design of lingulid
vs. other brachiopods? What superiority can a lungfish assert over a marlin or
tuna? In fact, since living fossils also (by traditional depiction) present such a
"primitive" or "archaic" look, the claim for optimality seemed specially puzzling.
The other obvious explanation, in a gradualistic and anagenetic world ruled
by conventional selection, held that living fossils had stagnated because they
lacked genetic variation, and therefore presented insufficient fuel for Darwinian
change. This more plausible idea seemed sufficiently intriguing that Selander et al.
(1970), in the early days of electrophoresis as a novel method for measuring
overall genetic variation, immediately applied the technique to Limulus, the
horseshoe crab—and found no lowering of genetic variability relative to known
levels for other arthropods. This negative pattern has held, and no standard lineage
of living fossils exhibits depauperate levels of genetic variability.
But punctuated equilibrium suggests another, remarkably simple, explanation
once you begin to think in this alternative mode—an insight that ranks in the
exhilarating, yet frustrating, category of obvious "scales falling from eyes"
propositions, once one grasps the new phrasing of a basic question. If evolutionary
rate correlates primarily with frequency of speciation—the cardinal prediction of
punctuated equilibrium—then living fossils may simply represent those groups at
the left tail of the distribution for numbers of speciation events through time. In
other words, living fossils may be groups that have persisted through geological
time at consistently and unvaryingly low species diversity. (Average species
longevity need not be particularly high,