612 THE STRUCTURE OF EVOLUTIONARY THEORY
As for the vernacular criteria previously discussed (see pp. 602-603), these
specifically evolutionary criteria teach us that organisms are not the only
individuals capable of acting as units of Darwinian selection. In particular, and
continuing to use species as a "type" example of individuality at higher levels, all
evolutionary criteria apply to the species as a basic unit of macro-evolution.
Species have children by branching (in our professional jargon, we even engender
these offspring as "daughter species"). Speciation surely obeys principles of
hereditary, for daughters, by strong constraints of homology, originate with
phenotypes and genotypes closer to those of their parent than to any other species
of a collateral lineage. Species certainly vary, for the defining property of
reproductive isolation demands genetic differentiation from parents and collateral
relatives. Finally, species interact with the environment in a causal way that can
influence rates of birth (speciation) and death (extinction).
As a further benefit for thus codifying the criteria of evolutionary
individuality, we can immediately cut through the foolishness surrounding several
distressingly common, but artlessly and rather thoughtlessly contrived, claims (or,
rather, loose metaphors) about the Darwinian character of large items in nature—
an attractive idea for many people, particularly for romantics and "new-agers" who
yearn for meaningful agency at the highest levels. We can dismiss these claims
because the object hypothesized as an agent of selection fails several crucial
requirements for designation as an evolutionary individual. As an obvious
example, many proponents of the so-called Gaia hypothesis wax poetic about the
earth and atmosphere as a homeostatic system robustly balanced by interaction
with life to secure and stabilize the conditions required by organisms for
diversification and geological persistence. Supporters often assume that such
functional coherence must make the earth sufficiently like an organism to merit
designation as a living entity. Some have even stated that the earth must therefore
be recognized as the largest and most inclusive product of Darwinian selection—or
even that the earth should, in fact, be viewed as a true Darwinian individual. This
woolly notion confuses a gut feeling about functionality or adaptive "optimality"
(for support of life) with the requirements of Darwinian agency. The earth does not
generate children, and did not arise by competitive prowess as the sole survivor
among defeated brethren (who must have died or been expelled, I suppose, from
the solar system long ago). Therefore, among a plethora of other reasons, the earth
cannot be construed as a Darwinian agent or unit of selection.
More plausibly, and more interestingly, communities and ecosystems have
sometimes been designated as potential units of selection. In this instance, at least,
a case could be conceived—for communities do maintain some functional
coherence, some boundaries (however loose), and some potential for splitting off
"daughter" communities with sufficient resemblance to a parent. But I can hardly
imagine a set of circumstances that would allow such ecological units to express
enough criteria of individuality to qualify for Darwinian agency. Communities are
not (for the most part) genealogically constructed or filiated. They can rarely
maintain sufficient coherence or persistence, for constituent species move in and
out in relative independence. Williams (1992,