Species as Individuals in the Hierarchical Theory of Selection 597
formulations—and he ultimately failed, as all others had. The logic of his argument
led Darwin to postulate higher-level selection at two crucial points (see pp. 127-
137)—to explain the evolution of altruism in human societies by interdemic
selection, and to encompass multiplication of species under his essential "principle
of divergence" by a partial appeal to species selection. If none of the most rigorous
and savvy early Darwinians could render evolution without some appeal to
selection at levels higher than individual organisms, shall we not tentatively
conclude that both the logic of the theory and nature's empirical record compel
such an expansion, and the attendant notion of hierarchy?
THE MEANING OF INDIVIDUALITY AND THE EXPANSION OF
THE DARWINIAN RESEARCH PROGRAMWe may agree with the strictest formulation of agency in Darwinian theory: natural
selection works by a struggle (actual or metaphorical) among individuals for
personal reproductive success. In other words, selection occurs when properties of
a relevant individual interact with the environment in a causal way to influence the
relative representation of whatever the individual contributes to the hereditary
make-up of future generations. If we place the theory's causal focus so squarely
upon individuals as agents, then we might suppose that Darwin's unitary
perspective must apply: all results at all evolutionary scales must cascade from the
causal process of selection among individuals, defined in the conventional
vernacular manner as the bodies of organisms.
But, as Hamlet said of the fears that prevent suicide (an adaptation, some
would no doubt argue, for keeping humans viable as Darwinian agents), "ay,
there's the rub." What is an individual? Are vernacular bodies the only objects in
nature that merit such a designation—especially when discrete "bodiness" doesn't
always define an unambiguous individual at the focal level of Darwin's intent (not
to mention the difficulties encountered in trying to characterize entities at levels
above and below bodies in the genealogical hierarchy of nature)?
For example, biologists spent more than a fruitless century trying to decide
whether the parts of siphonophores are "persons" in a colony or organs of an
organism—only to recognize that the question cannot be answered because both
solutions can justly claim crucial and partial merit (see Gould, 1984d). Are grass
blades or bamboo stalks bodies in their own right (as some aspects of functional
organization suggest), or parts (called ramets) of a larger evolutionary individual
(called a genet)? Do our feelings about definition shift when ramets become
spatially discrete and therefore look just like conventional bodies—as in the
parthenogenetic offspring of an aphid stem-mother (designated, in their totality, as
a single El, or evolutionary individual, by Janzen, 1977)? And what shall we do
with discrete bodies that maintain some genetic variation among themselves (and
cannot, therefore, form a set of identical ramets), but operate together as
differentiated items (analogs of organs)