Species as Individuals in the Hierarchical Theory of Selection 743
species level itself. But the new variation will often be adaptive at the species level
for two reasons: first, because species-level rather than organismic processes often
underlie the genesis of the variation; and second, because variation caused at the
organismic level will often be synergistic with species advantages, whereas
mutational variation rarely enjoys synergism with the benefits of organisms.
(4) The common synergism of organismal with species advantages produces a
powerful acceleration of macroevolution (Gould and Lloyd, 1999). Drives of
directional speciation (often based on organismal adaptation) frequently foster
species selection along the same pathway by accelerating the speciation rate or,
perhaps more commonly, by enhancing the longevity of species arising in the
direction of the drive. On the other hand, when organismal selection runs counter
to the interests of species, negative species selection may provide the only effective
higher-level force that can act as a governor to slow or stymie the trend—probably
a common feature in phylogeny, and previously given (in textbooks of my student
days, but now rarely used) the unfelicitous and unfortunate name of
"overspecialization."
As a final point and guide to understanding the essential role of the species-
individual in macroevolution, we must remind ourselves of the highly unusual
character of the individual conventionally (and usually unthinkingly) taken as a
paradigm for all evolutionary causality—the organism. If we view evolutionary
change as tripartite in causal nature—with drive, selection, and drift as the three
major modes—then we may say that the organism allows selection to reign nearly
supreme by "clearing out" the surrounding space of the other two processes. Drives
do not seem important at the organismal level, because drives emerge from below,
and organisms, as repeatedly emphasized in this chapter, work so effectively as
suppressors of lower-level selection. At the same time, drift produces limited
impact at the organismal level because population sizes tend to be too large in most
circumstances, and because the high degree of functional integration within
organisms grants a selective significance to nearly every part, thus lowering the
relative frequency of substantial neutrality in potential sites for drift. Therefore,
selection based on organismal properties reigns at this canonical level—thus
engendering the two great parochial prejudices of the strict Darwinian world view:
the adaptationist program as a guide to nearly all evolutionary phenomena, and the
virtual restriction of causality to natural selection working at the single level of
organisms (two of the three legs of Darwin's tripod in the terms of this book).
But when we turn to the species level, we find an interesting partnership
among the three causal forces of drive, selection and drift. Selection at the species
level does not "clear out" these surrounding forces. Drives from below exert great
influence in the phenomenon of directional speciation. Drift maintains similar
impact in both its major manifestations: as species drift for the transformation of
collectivities (clades); and as founder drift in differential proliferation or reduction
of subclades by accidents of propitious or limiting colonization. This absence of
"clearing out" denotes no failure or weakness