732 THE STRUCTURE OF EVOLUTIONARY THEORY
sense of the commanding potency of organismal selection rests upon the
conformity of Mendelian genetics to one of the cardinal prerequisites of Darwinian
systems (see Chapter 2): that the variation serving as raw material for natural
selection be "random" (with an operational meaning of "undirected towards
adaptive states," not "equally likely in all directions")—so that selection, rather
than biases inherent in variation, can become the "creative" force in evolutionary
change (see p. 144 for further discussion in a related context). This crucial
condition can be validated at the organismic level—not because mutations (and
other sources of genetic variation) are truly random in the mathematical sense, but
because mutation represents a process so different from natural selection, and
operating on material (the structure of DNA) so disparate from the bodies of
organisms (integrated tissues and organs), that we cannot postulate a reason why
favored directions of mutation should correspond in any way to the needs of
organisms.
But no comparable argument exists for any a priori expectation that the
analogous variation (among species within a clade) made available for species
selection should also be random with respect to the direction of a trend. Species do
not discourage drives among their parts (organisms), while organisms usually do
suppress directional variation at lower levels (because the proliferative "interests"
of individual genes and cell lineages generally run counter to the adaptive needs of
organisms). Moreover, the adaptive features of organisms often confer benefits
upon their species as well—as when species live longer because their well-
designed organisms prevail in competition. Therefore, we cannot defend an a
priori basis for asserting randomness in the variation that serves as raw material for
species selection.
This situation creates both a problem and a challenge for the analog of
Darwinism at the species level—for maximal efficiency of species selection does
demand undirected variability, and by the same classical argument originally
devised for the organismic level. The randomness of species-level variation with
respect to the direction of a trend therefore becomes a matter for empirical testing,
rather than a claim predictably flowing from the nature of materials and processes.
Such a test should also receive high priority for anyone interested in discovering
the frequency and strength of species selection in the explanation of evolutionary
trends.
For these reasons, Gould and El dredge (1977) formulated such a test under
the name of "Wright's Rule." We took our cue from a prescient statement by
Sewall Wright (1967) that the direction of speciation might be random with respect
to the origin of higher taxa, just as we consider mutation to be random relative to
the direction of natural selection. Wright's Rule, in our formulation, therefore
asserts either that drives of directional speciation do not exist at all in a given
situation (the strong version), or at least that any existing directional bias not occur
along the vector of an established trend (a weaker version, but fully adequate for
assertions of species selection). If Wright's Rule holds, then trends must be
attributed to differential proliferation of certain kinds of species (by selection or
drift), and not to any drives from within based on directional variation arising from