Species as Individuals in the Hierarchical Theory of Selection 615
The distinction of replicators and interactors
as a framework for discussion
Both leading founders of modern gene selectionism as a general view of evolution
(Williams, 1966; Dawkins, 1976) drew a crucial distinction between reproductive
units of heredity, and entities that interact with the environment to bias the
transmission of reproductive units into the next generation. Williams viewed nearly
all evolution as proceeding via genes as reproductive units, with adaptation of
organisms (the interacting entities) construed as a result—a duality that he usually
labeled (1966, p. 124 for example) as "genie selection and organic adaptation."
Dawkins (1976) agreed entirely, and drew a more colorful and explicit distinction
between "replicators," considered as units of selection and identified as genes—
and "vehicles," considered as merely passive repositories built by replicators for
their own purposes, and identified as bodies of organisms. In other words, both
Williams and Dawkins invoked a criterion of replication to identify genes as the
active and fundamental agents of natural selection.
In his 1980 review on "Individuality and selection," David Hull formalized
this distinction in a manner that has—quite usefully and properly in my view—
organized the professional discussion on units of selection ever since. Hull (1980,
p. 318) defined a replicator as "an entity that passes on its structure directly in
replication"; and an interactor as "an entity that directly interacts as a cohesive
whole with its environment in such a way that replication is differential." Hull then
defined selection with reference to both attributes: "a process in which the
differential extinction and proliferation of interactors cause the differential
perpetuation of the replicators that produced them."
Hull insisted that a causal account of selection must include both concepts
(1980, pp. 319-320): "Evolution of sorts could result from replication alone, but
evolution through natural selection requires an interplay between replication and
interaction. Both processes are necessary. Neither process by itself is sufficient.
Omitting reference to replication leaves out the mechanism by which structure is
passed from one generation to the next. Omitting reference to the causal
mechanisms that bias the distribution of replicators reduces the evolutionary
process to the 'gavotte of the chromosomes,' to use Hamilton's propitious phrase."
Later, Hull (1994, pp. 627-628) continued to espouse this view: "According to the
terminology I prefer, there are no units of selection because selection is composed
of two subprocesses—replication and interaction. Selection results from the
interplay of these two subprocesses. Genes are certainly the primary (possibly sole)
units of replication, whereas interaction can occur at a variety of levels from genes
and cells through organisms to colonies, demes, and possibly entire species."
I shall argue in this section that the causality of selection resides in
interaction, not in replication, and that the hierarchical model almost automatically
prevails once we accept this analysis of causality. Moreover, Hull's intuitions ran
in this direction from the start, for even while he insisted upon the "relevancy" of
both replication and interaction, Hull always acknowledged that