particular, and by these criteria, species must be construed not only as classes (as
traditionally conceived), but also as distinct historical entities acting as good
Darwinian individuals—and therefore potentially subject to selection. In fact, a full
genealogical hierarchy of inclusion—with rising levels of genes, cell lineages,
organisms, demes, species and clades—features clearly definable Darwinian
individuals, subject to processes of selection, at each level, thus validating (in logic
and theory, but not necessarily in the potency of actual practice in nature) an
extension and reformulation of Darwin's exclusively organismal theory into a fully
hierarchical theory of selection.
- The validity of the "interactor approach" to defining the mechanics of
selection, and the fallacy of the "replicator approach" expose, as logically invalid,
all modern attempts to preserve Darwinian exclusivity of level, but to offer an even
more reductionistic account in terms of genes, rather than organisms, as agents—
with organisms construed as passive containers for the genes that operate as
exclusive agents of natural selection. This false argument, based upon the true but
irrelevant identification of genes as faithful replicators, must be replaced by the
conceptually opposite formulation of a hierarchical theory of selection, with genes
identified as only one valid, and lowest, level in a hierarchy of equally potent, and
interestingly different, levels of Darwinian individuality: genes, cell lineages,
organisms, demes, species and clades. Replication identifies a valid and important
criterion for the crucial task of bookkeeping or tracing evolutionary change; but
replicators cannot specify the causality of selectionist processes, which must be
based upon the recognition and definition of interactors with environments. Even
Williams and Dawkins, the two leading exponents of exclusive gene selectionism,
have acknowledged and properly described the hierarchical causality of interaction
(while proferring increasingly elaborate and implausible verbal defenses of gene
selection in arguments about parallel hierarchies and Necker cubing of legitimate
alternatives rooted in criteria of replication vs. interaction). Thus, Williams and
Dawkins seem to grasp the validity of hierarchical selection through a glass darkly,
while still trying explicitly to defend their increasingly indefensible preferences for
exclusive gene selectionism. - The logic of hierarchical selection cannot be gainsaid, and even Fisher
admitted the consistency, even the theoretical necessity, while denying the
empirical potency, of species selection. Fisher based his interesting and powerful
argument on his assumption that low N for species in clades (relative to organisms
in populations) must debar any efficacy for species selection in a world of
continuous and gradualistic anagenesis rooted in organismal selection. However,
Fisher's argument, although logically tight, fails empirically because species tend
to be stable and directionally unchanging (however fluctuating) during their
geological lifetimes, and the theoretically "weaker" force of species selection may
therefore operate as the "only game in town" for macroevolution. The arguments
for potency of species selection are stronger than corresponding assertions for
interdemic selection (largely because species actively maintain their boundaries as
Darwinian individuals, whereas demes remain subject to breakup and invasion).
But, despite these intrinsic
Defining and Revising the Structure of Evolutionary Theory 73