902 THE STRUCTURE OF EVOLUTIONARY THEORY
(see Gould, 1970b) only restates the general principles of mathematical probability
for the specific case of temporal changes based on large numbers of relatively
independent components. And Williston's law of reduction and specialization in
modular segments may only record a structural constraint in random systems, thus
following the same principles as my previous argument about the expanding right
tail of complexity for life's totality. Suppose that, in overall frequency within the
arthropod clade, modular species (with large numbers of similar segments) and
tagmatized species (with fewer fused and specialized groupings of former
segments) always enjoy equal status in the sense that 50 percent of habitats favor
one design, and 50 percent the other. (I am, of course, only presenting an abstract
"thought experiment," not an operational possibility for research. Niches don't exist
independent of species.) But suppose also that, for structural reasons, modular
designs can evolve toward tagmatization, but tagmatized species cannot revert to
their original modularity—an entirely reasonable assumption under Dollo's law
(founded upon the basic statements of probability theory) and generalities of
biological development. Then, even though tagmatization enjoys no general
selective advantage over modularity, a powerful trend to tagmatization must
pervade the clade's history, ultimately running to completion when the last modular
species dies or transforms.
However, one of these older general rules has retained its hold upon
evolutionary theory, probably for its putative resolvability in more conventional
Darwinian terms of general organismic advantage: Cope's Law, or the claim that a
substantial majority of lineages undergo phyletic size increase, thus imparting a
strong bias of relative frequency to the genealogy of most clades—a vector of
directionality that might establish an arrow of time for the history of life.
A century of literature on this subject had been dominated by proposed
explanations in the conventional mode of organismic adaptation fueled by natural
selection. Why, commentators asked almost exclusively, should larger size enjoy
enough general advantage to prevail in a majority of lineages? Proposed
explanations cited, for example, the putative benefits enjoyed by larger organisms
in predatory ability, mating success, or capacity to resist extreme environmental
fluctuations (Hallam, 1990; Brown and Maurer, 1986).
The speciational reformulation of macroevolution has impacted this subject
perhaps more than any other, not because the theme exudes any special propensity
for such rethinking—for I suspect that almost any conventional "truth" of
macroevolution holds promise for substantial revision in this light—but because its
salience as a "flagship," but annoyingly unresolved, issue inspired overt attention.
Moreover, the conventional explanations in terms of organismal advantage had
never seemed fully satisfactory to most paleontologists.
The rethinking has proceeded in two interesting stages. First, Stanley (1973),
in a landmark paper, proposed that Cope's Law emerges as a passive consequence
of Cope's other famous, and previously unrelated, "Law of the Unspecialized"—
the claim that most lineages spring from founding species with generalized
anatomies, under the additional, and quite reasonable, assumption