624 THE STRUCTURE OF EVOLUTIONARY THEORY
individuals as interactors, causal agents, and units of selection; and the validation
of a hierarchical theory of natural selection based upon a principled understanding
that evolutionary individuals exist at several levels of organization—including
genes, cell lineages, organisms, demes, species, and clades.
D. S. Wilson has most vigorously championed this revival (Wilson, 1980,
1983), while his collaboration with philosopher E. Sober has produced a
particularly important paper and a subsequent book on the subject (Wilson and
Sober, 1994, with 33 accompanying commentaries and the authors' response;
Sober and Wilson, 1998). Wilson and Sober anchor their argument by insisting that
units of selection must be defined as interactors, not replicators.
I must raise only one mild quarrel with Wilson and Sober. I agree entirely that
units of selection must be denned as interactors, but I prefer a "looser" or "broader"
concept of interaction that fosters the proper identification of highest-level
individuals in species and clade selection. Wilson and Sober stress the "organism-
like" properties of interactors, and therefore make the confusing and regrettable
linguistic decision to use "individual" for conventional bodies, and "organism" as
the general name for a unit of selection at any hierarchical level; whereas I and
most biologists (see Gould and Lloyd, 1999) advocate a reversed terminology. In
characterizing the evolutionary principle of interaction, I would stress the potential
for rich panoply of emergent fitnesses, and for the consequent capacity of
plurifaction.
Their chosen stress on "organism-like" properties leads Wilson and Sober to
emphasize direct modes of interaction based on actual contact of sympatric
individuals—the old vision of two gladiators duking it out to the finish. But
interaction does not require physical contact. Interaction occurs between
individuals and environments, not necessarily between individual and individual.
The interaction must be able to yield plurifaction for causal reasons based on
properties that enhance differential reproductive success— but, again, competing
individuals need not interact directly with each other. Rather, to speak of selection,
competing individuals only need to plurify at different relative rates based on
similar causal interactions with environments. But the environments may be
spatially separate and broadly defined. This issue does not often arise at the
traditional level of Darwin's chosen evolutionary individuals—that is, organisms.
But higher-level individuals, particularly species and clades, do often compete
without contact—and our notion of units of selection must include this important
mode of interaction.
Several thoughtful biologists have stressed this point, and I have compiled a
small file of such statements. I shall present here only the forceful argument of
Williams (1992, p. 25), who has changed his view substantially since formulating
the theory of gene selectionism in 1966:
There are many further questions on the meaning and limits of clade
selection. One issue is whether the populations that bear the gene pools
need be in ecological competition with each other. I believe that this is