Species as Individuals in the Hierarchical Theory of Selection 729
A-Daughters and 50 percent B-Daughters. (If we posit random mortality of a given
percentage of species before they split into their daughters, as in Fig. 8-6, then B-
Species will eventually be entirely eliminated, and A-Species will become fixed in
the clade.)
A first kind of example would not disturb the tranquility of any committed
adaptationist, for a functionalist theme translates well across the levels. Suppose
that Cope's Rule were true in the classical sense—it is not, by the way (Stanley,
1973; Jablonski, 1987, 1997; McShea, 1994; Gould, 1988b, 1997b, and pp. 902-
905 of this book)—and that organismal selection always favored size increase
because big organisms prevail in competition. A-Species are large and B-Species
are small; A's only give rise to other A's, while B's give rise either to A's (given the
pervasive advantage of increasing size), or to B's at equal frequency (for small size
may still be favored in some habitats of the clade). The strong Cope's-Rule trend in
the clade occurs by directional speciation. The adaptationist theme prevails at both
levels. Average organisms in the clade become larger because bigger is better; and
species increase in average body size because their parts (organisms) do better at
larger size. (No species-level trait exists in regulating this trend, and the entire
phenomenon arises by conventional organismal selection based on advantages of
increased body size.)
But a second kind of example—undoubtedly quite common in evolution—
would perturb a strict adaptationist by translating selection at the organismic level
to regulation of the cladal trend by constraint. Suppose now—and such an
explanation has been urged as an alternative to species selection for the increase of
nonplanktotrophic species within Tertiary clades of gastropods (Strathmann, 1978,
1988)—that a molluscan clade begins with an equal number of species of
nonplanktotrophs (A-Species) and plankotrophs (B-Species). Planktotrophic larvae
stay aloft through the motion of complex ciliary bands that beat in concert.
Selection pressures for nonplanktotrophy lead to loss of these bands, and
consequent benthic development of a maternally protected brood. Plankotrophs can
always, in principle, convert to nonplanktotrophy because the bands can be lost;
but the transition cannot proceed in the other direction because ciliary bands can't
be reconstituted once they have disappeared in evolution (see Gould, 1970b, on the
proper meaning of Dollo's Law of irreversibility in evolution).
The origin of each species may be governed entirely by the conventional route
of adaptation based on natural selection of organisms. But a structural limitation in
possible directions of change produces the cladal trend by directional speciation
towards increasing frequency of nonplanktotrophic species—for a planktotrophic
parent species can generate either planktotrophic or nonplanktotrophic daughters,
while a nonplanktotrophic parent can only produce nonplanktotrophic daughters.
The numerical situation corresponds exactly with Figure 8-6 and the previous
example based on Cope's Rule (with A-Species now read as nonplanktotrophs, and
B-Species as planktotrophs), but the explanation at the cladal level differs
crucially—for the trend arises by structural constraint upon possible directions of
change, not from