802 THE STRUCTURE OF EVOLUTIONARY THEORY
the fossil record." Futuyma directly follows this statement with the key implication
of punctuated equilibrium for the explanation of evolutionary trends: "Long-term
anagenetic change in some characters is then the consequence of a succession of
speciation events."
Later in his article, Futuyma (p. 467) explicitly links speciation with sufficient
stability (individuation) for macroevolutionary expression: "In the absence of
reproductive isolation, differentiation is broken down by recombination. Given
reproductive isolation, however, a species can retain its distinctive complex of
characters as its spatial distribution changes along with that of its habitat or niche.
.. Although speciation does not accelerate evolution within populations, it
provides morphological changes with enough permanence to be registered in the
fossil record. Thus, it is plausible to expect many evolutionary changes in the fossil
record to be associated with speciation." And, at the end of his article, Futuyma (p.
470) notes the crucial link between punctuated equilibrium and the possibility of
sustained evolutionary trends: "Each step has had a more than ephemeral existence
only because reproductive isolation prevented the slippage consequent on
interbreeding with other populations... Speciation may facilitate anagenesis by
retaining, stepwise, the advances made in any one direction... Successive
speciation events are the pitons affixed to the slopes of an adaptive peak."
I hope that Futuyma's simple yet profound insight may help to heal the
remaining rifts, thereby promoting the integration of punctuated equilibrium into
an evolutionary theory hierarchically enriched in its light.
CRITIQUES BASED UPON SUPPOSED FAILURES OF
EMPIRICAL RESULTS TO AFFIRM PREDICTIONS
OF PUNCTUATED EQUILIBRIUMI shall treat the specifics of this topic primarily in the next section on "the data of
punctuated equilibrium." But the logic of this chapter's development also requires
that I state the major arguments and my responses in this account of principal
critiques directed at the theory—for the totality of attempted rebuttals has not only
posited theoretical objections in an effort to undermine the theory's logic or
testability (as discussed in the first two parts of this section), but has also
proceeded by accepting the theory's program of research as valid, and then arguing
that the bulk of data thus accumulated refutes punctuated equilibrium empirically. I
shall summarize discussion on the two major strategies pursued under this rubric:
refutation by accumulation of important cases, and rejection by failure of actual
data to fit models for predicted phylogenetic patterns.
Claims for empirical refutation by cases
PHENOTYPES. Despite some early misunderstandings, long since resolved by all
parties to the discussion, we recognize that no individual case for or against
punctuated equilibrium, however elegantly documented, can serve as a "crucial
experiment" for questions in natural history that must be decided