Punctuated Equilibrium and the Validation of Macroevolutionary Theory 805
This central problem of early Mendelian genetics could never be addressed— even
to counter the worst abuses of biological determinism and social Darwinism—
because biologists could not generate random samples of genes, and could
therefore only overestimate average distances by ignoring the unknowable
invariant genes among races, while studying the (potentially small) fraction
capable of recording differences among groups. With electrophoretic techniques,
and the attendant generation of a random sample with respect to potential
variability, geneticists soon calculated the average genetic differences among races
as remarkably small and insignificant—a conclusion of no mean practical
importance in a xenophobic world. Similarly, a truly random sample (with respect
to the distribution of anagenetic rates) might show predominance for stasis, even if
previous studies (with their strong bias for preselection of variable species) had
generally affirmed gradualism.
I am encouraged to accept the probable validity of this argument by the
important study of Wagner and Erwin (1995), who used the different and
comprehensive technique of compiling full cladograms for two prominent Neogene
families of planktonic forams: Globigerinidae and Globorotaliidae. In applying a
set of methods for inferring probable evolutionary mode from cladistic topology
(see full discussion and details on pp. 820-822), they found that, in both families,
branching speciation in the mode favored by punctuated equilibrium (divergence
of descendants with survival of ancestors in stasis) vastly predominated over the
origin of new species by anagenetic transformation. Thus, the literature's apparent
preference for anagenesis in tabulated studies of individual lineages may only
record an artifact of biased selection in material for research.
- Even if gradualism truly does prevail in planktonic forams, we could not
infer that the observed predominance of punctuated equilibrium in marine Metazoa
must therefore reflect the artifact of an imperfect geological record. The difference
might record a characteristic disparity between the taxa, not a general distinction in
quality of geological evidence between deep oceanic cores and conventional
continental sequences—a proposition defended in the third argument, just below.
The deep oceanic record may usually be more complete, but the subset of best
cases from conventional sequences surely matches the foram data in quality—and
convincing studies of punctuated equilibrium and gradualism generally use these
best records. Thus, the subset of most adequate metazoan examples should match,
in quality of evidence, the usual records of forams from oceanic cores. - A third argument completes the trio of logical possibilities (all partially
valid, I suspect, though I would grant most weight to this third point) for denying
that a currently recorded maximal frequency of gradualism for planktonic
foraminiferal lineages casts doubt on the general importance of punctuated
equilibrium in evolution. The first argument attributes an apparent high frequency
to biased sampling in the preselection, for rigorous study, of lineages already
highlighted by taxonomic experts for suspected gradual change. The second and
third arguments, on the other hand, hold that if high frequency truly characterizes
this group, no general rebuttal of punctuated