620 THE STRUCTURE OF EVOLUTIONARY THEORY
passage and sufficient persistence—the properties most strikingly exhibited by
genes. But evolutionary individuals, to act as units of selection, must also display
other properties that genes do not generally possess. In particular, a unit of
selection must interact "directly ... as a cohesive whole with its environment in
such a way that replication is differential"—to quote Hull's definition once again
(1980, p. 318).
But in sexual organisms, and in other higher-level individuals, genes do not
usually interact directly with the environment. Rather, they operate via the
organisms that function as true agents in the "struggle for existence." Organisms
live, die, compete and reproduce; as a result, genes move differentially to the next
generation.
Of course genes influence organisms; one might even say, metaphorically to
be sure, that genes act as blueprints to build organisms. But such statements do not
substantiate the critically necessary claim that, therefore, genes interact directly
with the environment when organisms struggle for existence. The issue before
us—the venerable problem of "emergence"—is largely philosophical and logical,
and only partly empirical. Genes would interact directly only if organisms
developed no emergent properties—that is, if genes built organisms in an entirely
additive fashion, with no nonlinear interaction among genes at all. In such a
situation, organisms would be passive repositories, and genes could be construed
as units of selection—for anything done by organisms could then be causally
reduced to the properties of individual genes.
This aspect of the question must be decided empirically. But the issue is also
quite settled (and was never really controversial): organisms are stuffed full of
emergent properties; our sense of organismic functionality and intentionality
largely arises from our appreciation of these emergent features. Thus, since genes
interact with the environment only indirectly through selection upon organisms,
and since selection on organisms operates largely upon emergent characters, genes
cannot be units of selection when they function in their customary manner as
faithful and differential replicators in the process of ordinary natural selection
among organisms. Dawkins's metaphors of selfish genes and manipulated
organisms may be colorful, but such images are also fatefully misleading because
Dawkins has reversed nature's causality: organisms are active units of selection;
genes, while lending a helping hand as architects, remain stuck within these
genuine units.
THE THEORY-BOUND NATURE OF CONCEPTS AND DEFINITIONS. We are drawn to
the faithfulness of gene replication, especially when compared with the contrasting
transiency of sexual organisms, who must disaggregate to reach the next
generation. We might therefore assume that genes become primary candidates for
units of selection as a consequence of their potential immortality, while organisms
fall from further consideration by the brevity of their coherent lives.
"Sufficient stability" surely ranks as an important criterion for the "evolutionary
individuality" required of a "unit of selection." But, in Darwinian theory and the
search for units of selection, "sufficient" stability can only be defined as enough
coherence to participate as an unchanged individual in the causal process of