Punctuated Equilibrium and the Validation of Macroevolutionary Theory 931
such a discovery can only lie in extrapolating this particular and strongly scale
bound cause to debunk a different mechanism previously proposed to explain the
same pattern at another level—rather than exploring the more fruitful and
integrating hypothesis that a genuine basis for meaningful similarity in pattern
might reside in homologous structural constraints that channel different causes to
similar outcomes at the two distinct scales.
For example, an excellent science reporter for the New York Times
erroneously argued that punctuations caused by long waiting times between
rapidly-sweeping, favorable mutations in bacterial anagenesis on a scale of months
should lead us to reinterpret the speciational breaks of punctuated equilibrium (at
geological scales) as similarly caused by quick and simple genetic changes! "The
finding that all it takes is a few mutations and a little natural selection to generate
punctuated evolution comes as a surprise. Researchers say numerous theories that
are considerably more complex have been put forth to explain what might produce
the punctuation seen in the fossil record. If bacteria are any indication, the rapid
evolution documented in the fossil record might be the product of a very few
simple, if quick, genetic changes" (Yoon, 1996).
But R. E. Lenski, the chief scientist in the bacterial study (Elena, Cooper, and
Lenski, 1996), properly sought commonality with punctuated equilibrium in the
domain of homologous reasons for punctuational patterns. Recognizing the
disparities in scale, and the different causes thus implied, they rightly declined to
apply the term-punctuated equilibrium to their findings. Instead, they invoked the
general term for the pattern itself as the title for their paper (Elena, Cooper, and
Lenski, 1996): "Punctuated evolution caused by selection of rare beneficial
mutations."
PUNCTUATION BELOW THE SPECIES LEVEL. I have, at several points in this
and the preceding chapter, discussed various empirical and theoretical studies that
validate the pattern of substantial stability followed by rapid peak shifts in the
anagenetic transformation of single populations during the microevolutionary time
of potential human observation (see p. 877). I have also urged (to reiterate the
theme of the preceding section) that such an important conclusion should not be
read as an argument that punctuated equilibrium holds no interest for evolutionary
theory because ordinary population genetics can produce patterns of stasis and
punctuation—a common but erroneous claim rooted in the misinterpretation of
punctuated equilibrium as a saltational theory in ecological time. Rather, this
small-scale anagenetic conclusion for another domain of size and time should be
read as welcome confirmation—based on causes different from the generators of
punctuated equilibrium at a larger scale—for the broader claim that punctuational
patterns may be common and robust across several spatial and temporal realms in
nature.
But the most impressive affirmations of punctuational patterns at scales below
punctuated equilibrium have emerged, in recent years, from a domain unparalleled
(and unmatchable) for richness of empirical data on evolution