616 THE STRUCTURE OF EVOLUTIONARY THEORY
the classical argument for multiple levels of selection only invokes interactors. He
wrote in his original paper (1980, p. 325): "In most cases when biologists argue
that entities more inclusive than single genes function in the evolutionary process,
they have interaction in mind." And Hull (1994, p. 628) directly followed his
defense of duality (quoted just above) with this sentence: "The units-of-selection
controversy concerns levels of interaction, not levels of replication." I shall defend
and develop Hull's intuition in the rest of this section. Only interactors can be
deemed causal agents in any customary or reasonable use of this central term.
Replicators are important in evolution, but in a different role as items for
bookkeeping. Replicators are not causal agents. If causality resides in interactors,
and interactors at several levels rank as legitimate evolutionary individuals, then
the hierarchical theory of selection becomes unassailable as a coherent logical
structure, subject to the ultimate scientific test of empirical verification (or
invalidation) in nature.
Faithful replication as the central criterion for the gene-centered
view of evolution
As noted above, both Williams and Dawkins chose to define units of selection as
replicators rather than interactors. I shall explain under argument three why I am
confident that they made the wrong choice—thus committing the fruitful "Pareto
error" discussed at the outset of this section. Having thus decided, and correctly
understanding that selection can only work on "individuals" as previously defined,
what replicating individuals would Williams and Dawkins then designate as units?
We all know that they chose genes as fundamental—and effectively
exclusive—replicators, and therefore as the unit of selection in Darwinian theory
(in maximal contrast with the hierarchical theory of multiple, simultaneously
acting levels, as defended in this book). I will discuss the stated reasons for their
choice, but I cannot know the deeper motivations of their philosophical and
psychological preferences. I strongly suspect that they, and all defenders of strict
gene selectionism, feel drawn to the traditional reductionism of science. They
understood that Darwin himself went as far as he could in this direction, by
breaking down the Paleyan edifice of highest-level intentionality (God himself) to
the lowest level then practical—organisms struggling for reproductive success (see
Chapter 2). They also recognized that this breakdown had produced revolutionary
consequences for Western thought, particularly in reconceptualizing all perceived
natural "benevolence" (especially the good design of organisms and the harmony
of ecosystems) as a side-consequence of struggle for personal success among
lowest-level individuals, rather than as an explicit intention of a loving and
omnipotent deity. I imagine that the more thoughtful gene selectionists then
worked by analogy, reasoning that if they could break causality down even further,
below the level of the organism, similarly interesting, and perhaps revolutionary,
consequences might follow. I can't gainsay either the intuition or the ambition—
but I can fault the resulting argument for an erroneous choice of both category and
of level.