The Structure of Evolutionary Theory

642 THE STRUCTURE OF EVOLUTIONARY THEORY
be interactors in the much more restricted status of one legitimate level in an extensive
hierarchy, as discussed on pp. 689-695)—and interactors, not replicators, are units of
selection in the causal sense.
Williams therefore tried an interesting gambit. He admitted that interactors form
a hierarchy of evolutionary individuals at several levels, and that these interactors are
units of selection in our usual sense of material entities participating in a causal
process. These interactors build a material hierarchy—and gene selectionism cannot
apply to this legitimate domain. Williams therefore established a different and parallel
hierarchy for abstract units of information (as opposed to material entities)—and he
construed genes as basic "units of selection" in this alternative and parallel domain,
which he called codical (the adjectival form of codices, the plural of codex, his term
for a single unit
This interesting idea of parallel hierarchies to separate the replicative and interactive criteria
of evolutionary individuality originated with Eldredge (1989; see also Vrba and Eldredge, 1984),
who spoke of genealogical and economic hierarchies. The scheme continues here with
Williams's similar distinction of material and codical systems. I find the idea of dual hierarchies
both interesting and challenging, but ultimately flawed and counterproductive in the introduction
of unnecessary complexity. (My rejection of this scheme defines my only major difference with
my closest colleague Niles Eldredge, who has worked with me for 25 years on problems of
macroevolutionary theory.)
Eldredge's "economic" and Williams's "material" hierarchies include the interactors defined as
proper units of selection in this book—and also in Wilson and Sober (1994), and (by unintended
verbal admission, though not explicitly) by such gene selectionists as Dawkins and Williams, as
I have shown throughout this section. (Eldredge calls this hierarchy "economic" to stress the
doing and dying of such entities in nature's ecosystems.) Eldredge's "genealogical" and
Williams's "codical" hierarchies express the concept of replication (as nonmaterial units of
information for Williams, but as an alternative hierarchy of replicating material entities for
Eldredge).
I find the framework of dual sequences unnecessarily complex and divisive because a single
theme unites our search to define units of selection, and a single hierarchy expresses this theme
in the best and clearest way. Units of selection must be evolutionary individuals by the criteria
outlined on pages 608-613. Above all, such individuals must be interactors in order to function
as units of selection in a causal process. They must also possess a mechanism of plurifaction—
that is, interactors must be able to bias the heredity of subsequent generations towards more of
their own contribution, however these contributions be packaged. This need for plurifaction
underlies our sense that replication plays a vital role in evolutionary individuality—a role
sufficiently important to be mistaken as causal and primary by gene selectionists, or at least to
warrant a separate hierarchy (by Eldredge). But I raise two points to obviate the need for a
separate hierarchy of replicators: (1) replication (or some other form of hereditary passage)
constitutes only one of several necessary criteria for defining evolutionary individuality; and (2)
this criterion of hereditary passage only demands that interactors possess a means of plurifaction;
faithful replication represents one style of hereditary passage, but not a necessary mode for
attribution of evolutionary individuality or designation as a unit of selection. Sexual organisms
plurify by disaggregation and differential passage of genes; other kinds of evolutionary
individuals plurify by faithful passage.
We should formulate a single hierarchy—call it material, genealogical, or perhaps simply
evolutionary—composed of interactors with adequate modes of plurifaction. These evolutionary
individuals build a hierarchy of inclusion, with each higher level encompassing the individuals
beneath as parts. Most units in Eldredge's parallel hierarchies appear in both his economic and
genealogical arrays—and therefore represent the evolutionary individuals we seek for a single
hierarchy—for these are the entities that possess both the interactive (economic) and hereditary
(genealogical) properties required of any evolutionary individual.