Punctuated Equilibrium and the Validation of Macroevolutionary Theory 791
punctuated equilibrium by our critics (but largely irrelevant to our theory), that
nearly all events of speciation produce an increment of macroevolutionary change.
This conclusion flows from elementary logic, not from empirical science. The
argument that all B comes from A does not imply that all A leads to B. All human
births (at least before modern interventions of medical technology) derived from
acts of sexual intercourse, but all acts of intercourse don't lead to births.
To draw a more relevant analogy: in the strict version of Mayr's peripatric
theory of speciation, nearly all new species arise from small populations isolated at
the periphery of the parental range. But the vast majority of peripheral isolates
never form new species; for they either die out or reamalgamate with the parental
population. Similarly, most new species may never be recorded in the fossil record;
but, if the theory of punctuated equilibrium holds, when changes do appear in
lineages of fossils, speciation provides the source of input in a great majority of
cases. Thus, most speciation could be cryptic (and unknowable from fossil
evidence), while effectively all macroevolutionary change still arises from the
minority of speciation events with phenotypic consequences. Just as peripheral
isolates might represent "the only game in town" for forming new species (though
few isolates ever speciate), cladogenetic speciation may be "the only game in
town" for inputting phenotypic change into macroevolution (though few new
species exhibit such change).
THE TREATMENT OF INELUCTABLE NATURAL BIAS IN SCIENCE. In an ideal
world—the one we try to construct in controlled laboratory experiments—no
systematic bias distorts the relative frequency of potential results. But the real
world of nature meets us on her own terms, and we must accept any distortions of
actual frequencies that directional biases of recording or preservation inflict upon
the archives of our evidence. At best, we may be able to correct such biases if we
can make a quantitative estimate of their strength. (This general procedure, for
example, has been widely followed to correct the systematic under measurement of
geological ranges imposed by the evident fact that observed first and last
occurrences of a fossil species can only provide a minimal estimate for actual
origins and extinctions, for the observed geological range of a species must be
shorter (and at least cannot be longer) than the actual duration. Studies of "waiting
times" between sequential samples within the observed range, combined with
mathematical models for constructing error bars around first and last occurrences,
have been widely used to treat this important problem—see Sadler, 1981; Schindel,
1982; Marshall, 1994.)
Often, however, we can specify the direction of a bias, but do not know how
to make a quantitative correction. In such cases, the sciences of natural history
must follow a cardinal rule: if the direction of bias coincides with the predicted
effect of the theory under test, then researchers face a serious, perhaps
insurmountable, problem; but if a systematic bias works against a theory, then
researchers encounter an acceptable impediment—for if the theory can still be
affirmed in the face of unmeasurable biases working against a favored explanation,
then the case for the theory gains strength.