weaknesses and problems, interdemic selection has now been empirically validated
as an important force in evolution—thus strengthening a prima facie case for the
even greater importance of species selection in macroevolution.
- Two theoretical resolutions and clarifications have established both a
sound theoretical basis, and a strong argument for the empirical potency, of species
selection as an important component of macroevolution: first, the recognition of
differential proliferation rather than downward effect as the most operational
criterion for defining and recognizing species selection; second, the
acknowledgment that emergent fitnesses under the interactor approach, rather than
emergent features treated as active adaptations of the species, constitute the proper
criterion for identifying species selection. The former insistence upon emergent
features (by me and other researchers, and in error), while logically sound and
properly identifying a small subset of best and most interesting cases, relegated the
subject to infrequent operational utility, and thus to relative impotence. The proper
criterion (under the interactor approach) of emergent fitness universalizes the
subject by permitting general identification in the immediacy of the current
mechanics of selection, and not requiring knowledge—often unavailable given the
limits of historical archives—of adaptive construction and utility in ancestral
states. - The six levels recognized for convenience, and not accompanied by any
claim of completion or exclusivity—gene, cell lineage, organism, deme, species
and clade—feature two important principles that make the theory of hierarchical
selection so different from, while still in the lineage and tradition of, exclusivistic
Darwinian organismal selection. First, adjacent levels may interact in the full range
of conceivable ways—in synergy, orthogonally, or in opposition. Opposition has
been stressed in the existing literature, but only because this mode is easier to
recognize, and not for any argument of greater importance in principle. Second, the
levels operate non-fractally, with fascinating and distinguishing differences in
mode of functioning, and relative importance of components, for each level. For
example, the different mechanisms by which organisms and species maintain their
equally strong individuality dictate that selection should dominate at the
organismal level, while selection, drift, and drive should all play important and
balanced roles at the species level. - To cite just one difference (from conventions of the organismal level) for
each nonstandard level, and to make the key point about distinctiveness of levels in
an almost anecdotal manner: random change may be most prominent in relative
frequency at the level of the gene-individual; true gene selection also plays an
important, if limited, role (largely in the mode that has been given the unfortunate
name—for its implication of opposition, almost in ethical terms, to the supposed
standard of proper organismal selection—of "selfish DNA"); however, the
Dawkinsian argument for exclusivity of genie selection only records the confusion
of a preferred level of bookkeeping with an erroneous claim for a privileged locus
of selection. Selection among cell-lineages, although ancestrally important in the
evolution of multicellular organisms, has largely been suppressed by the
organismal level in the interests
74 THE STRUCTURE OF EVOLUTIONARY THEORY