Punctuated Equilibrium and the Validation of Macroevolutionary Theory 813
diversity through time and space in major taxonomic groups—and then devise tests
to distinguish among contrasting causes: anagenetic vs. cladogenetic; gradual vs.
punctuational. If certain well-defined patterns can only be generated, say, by
branching speciation rather than by anagenetic transformation (or vice versa, of
course), then we can use the fit of broad results with distinctive models, rather than
minute documentation on a case-by-base basis, to establish the relative frequency
of punctuated equilibrium.
In an important study, for example, Lemen and Freeman (1989) investigated
"the properties of cladistic data sets from small monophyletic groups (6- 12
species) ... using computer simulations of macroevolution" (p. 1538). They
contrasted the differing outcomes of data generated under anagenetic gradualism
vs. punctuated equilibrium, and then examined cladograms of extant monophyletic
groups for consistency with these "abstract, end-member" alternatives. They
claimed better support for gradualism, but several flaws in their logic and data
render their conclusions moot, much as their pioneering approach may be
applauded and recommended for further study.
Lemen and Freeman tested actual data against three modelled differences between
cladograms generated by gradualism vs. punctuated equilibrium.
- Punctuated equilibrium should produce a strongly positive correlation
between number of branching points and apomorphies of species—because change
occurs at speciation and does not further correlate with passage of time per se.
Lemen and Freeman's models did affirm this expected result, and real data,
somewhat ironically, revealed "higher correlations of apomorphies and branch
points than could be explained by either mode of macroevolution" (p. 1549). But
the authors rejected an interpretation of this information as favorable to punctuated
equilibrium because anagenesis, under certain conditions, also yields positive
correlations, and because high correlations can also arise artificially by errors in
establishing cladograms: "a consistent error in polarity can profoundly affect the
correlation of total apomorphies and branch points" (p. 1551). Fair enough, but
Lemen and Freeman are not nearly so circumspect when equally flawed data seem
to favor their preferred alternative of gradualism. - The modal (but not the mean or median) number of autapomorphies will
always be zero under strict punctuated equilibrium. This odd-sounding situation
arises because, in the cladograms, an event of branching produces a daughter
species with some autapomorphies and a persisting parental species remaining in
stasis with none. With no change except at branching points, the value of zero
autapomorphies must remain most common across all species on the cladogram.
Under gradualism, autapomorphies simply accumulate through time, whatever the
pattern of branching, so zero should not mark a preferred or particularly common
value.
Lemen and Freeman never found a mode of zero autapomorphies in real data,
and therefore rejected punctuated equilibrium as a predominant style of evolution.
But had they pursued explanations based on artifacts (as they did so assiduously
when the data seemed to favor punctuated equilibrium), they would have realized
that taxonomic practice precludes the definition (or even