Punctuated Equilibrium and the Validation of Macroevolutionary Theory 817
for low species numbers, if consistently maintained without geological bursts of
radiation, will yield the full effect.) Such groups cannot be common—for
consistently low diversity makes a taxon maximally subject to extinction in our
contingent world of unpredictable fortune, where spread and number represent the
best hedges against disappearance, especially in episodes of mass extinction—but
every bell curve has a left tail.
This explanation holds remarkably well, and probably provides a basic
explanation of "living fossils." Such groups are neither mysteriously optimal, nor
unfortunately devoid of variability. They simply represent the few higher taxa of
life's history that have persisted for a long time at consistently low species
number—and have therefore never experienced substantial opportunity for
extensive change in modal morphology because species provide the raw material
for change at this level, and these groups have never contained many species.
Westoll (1949), for example, published a classic study, summarized again and
again in treatises and textbooks (Fig. 9-12), showing that lungfishes evolved very
rapidly during their early history, but have stagnated ever since. The literature
abounds in hypothesized explanations based on adaptation and ecological
opportunity in an anagenetic world. The obvious alternative stares us in the face,
but rises to consciousness only when theories like punctuated equilibrium
encourage us to reconceptualize macroevolution in speciational terms: in their
early period of rapid evolution, lungfishes maintained high species diversity, and
could therefore change quickly in modal morphology. Their epoch of later
stagnation correlates perfectly with a sharp reduction of diversity to very low
levels (only three genera living today, for example) with little temporal fluctuation
in numbers—thus depriving macroevolution of fuel for selection (at the species
level), and relegating lungfishes to the category of living fossils.
A breakthrough in the application of quantitative modelling to cladistic
patterns of evolution directly recorded in the fossil record has been achieved by
Wagner (1995 and 1999) and Wagner and Erwin (1995). These authors show, first
of all, the pitfalls of working only with cladistic information from living
organisms, and they illustrate the benefits of incorporating stratophenetic data from
the fossil record into any complete analysis (see Wagner and Erwin, 1995, pp. 96-
98, in a section entitled "why cladistic topology is insufficient for discerning
patterns of speciation"). They then build models based on three alternative modes
of evolution, and characterize the differences in cladistic pattern expected from
each: anagenetic gradualism, speciation by "bifurcation" (where, after branching,
the two descendant species both accumulate differences from an ancestor then
recorded as extinct), and speciation by "cladogenesis" (where one daughter species
arises with autapomorphic differences, but the ancestral species persists in stasis).
Cladogenesis is usually defined—both in this book and in the evolutionary
literature in general—as any style of evolution by branching of lineages rather than
by transformation of a single lineage (anagenesis). Wagner and Erwin restrict the
term "cladogenesis" to the mode of speciation predicted by punctuated
equilibrium—