The Structure of Evolutionary Theory

Species as Individuals in the Hierarchical Theory of Selection 741

force of directional speciation can greatly enhance and channel cladal trends by
working synergistically with such species-level modes of change as species
selection.
Perhaps the most important positive constraint, acting similarly at both levels,
lies in the large size of "exaptive pools" (see full discussion in Chapter 11), or
nonrandom variation made available through evolutionary processes acting on
other features or at other levels, but later exploitable by organisms or species for
their own exaptive benefits. The redundancy supplied by genetic duplication for
organismal flexibility serves as the classic illustration of this phenomenon at the
traditional level of natural selection. The exaptive pool of the species level may
become even larger because species do not suppress lower levels of change, while
these genie and organismal directionalities frequently act in synergism with
advantages at the species level (see Gould and Lloyd, 1999, for a detailed
development of this argument).

Summary comments on the strengths of species selection and its

of Change and its Interaction with Other Macroevolutionary Causes

Species selection, the Darwinian analog at this higher level, but by no means the
only irreducible force of macroevolutionary change, differs from conventional
natural selection at the organismic level both in character and in general strength.
The major aspect of character—as I have emphasized throughout this chapter in
stressing the non-fractality of hierarchical levels—lies in the potency of species
selection for governing "what species do." Species selection does not, and cannot,
build the complex adaptive phenotypes of organisms, but this common statement
only recognizes the general nature of hierarchical organization and does not
represent a fair criticism of the efficacy of species selection, despite the claims of
Dawkins (1982) and others (see p. 711 for a discussion of this point).
The primary force of species selection lies in its power to promote trends
within clades, and to regulate the waxing and waning of differential species
diversity within and among clades through time. The influence of species selection
upon trends will also be enhanced because this process not only builds trends in
species-level characters directly, but also establishes correlated trends in any
character of the organismal phenotype that either helps to determine the species-
level property, or merely hitchhikes upon the trend by linkages of homology within
the phylogenetic structure of evolutionary trees—a very common phenomenon, as
Raup and Gould, 1974, showed in theory and practice. This insight about trends,
which I shall explore more thoroughly in the next chapter (pp. 886-893), may
provide a key for explaining one of the most puzzling phenomena in
paleontology—persistent and pervasive cladal trends (such as decreasing stipe
number in graptolites, or increasing symmetry of crinoidal cups) that have defied
all attempts at explanation in traditional terms of biomechanical advantages to
organisms.
As for general strength, species selection (in primary comparison with the
traditional level of Darwinian natural selection on organisms) includes certain