610 THE STRUCTURE OF EVOLUTIONARY THEORY
world than the Darwinian shift to variation among members of a population as an
ultimate and irreducible reality (see Mayr, 1982b; Gould, 1996a)— a reversal of
the old Platonic notion that essences (approximated empirically by measuring
mean values, or by trying to construct an abstract ideal form and then searching for
a closest actual embodiment) define the nature of things, and that variation among
actual individuals (organisms in populations, in our most relevant example) can
only be construed as "accidental," and judged by relative departure from a
materially unattainable ideal.
Heredity and reproduction work in concert with variation to empower
Darwinian selection in genealogically recognizable lineages. The failure of any
criterion debars Darwinian evolution as a genealogical process. An absence of
reproduction, for example, enforces an oddly limited form of "evolution" restricted
to rules (or vagaries) of change within one or a number of individuals, all
separately constructed at the outset. Vernacular usage, in fact, does apply the term
"evolution" to some nongenealogical systems of this sort—as in the "evolution" of
stars along the H-R sequence. But the causes of such systematic temporal changes,
unfolding predictably under laws of nature (and not by the contingencies of
variational history), differ so profoundly from Darwinian evolution that we really
should insist upon different words for these maximally disparate modes of history
(Gould, 2000a). (A great burden of misunderstanding, in both popular and
professional cultures, must be ascribed to our confusing use of common
terminology for such different causes. Many interested laypeople feel that
biological evolution must unfold by internal necessity just as stars follow their
predictable sequences and as galaxies expand following the big bang. And many
professional evolutionists, suffering from the common affliction of physics envy,
and immured in the reductionistic biases of Western scientific culture, have tried to
find progressive patterns directly imposed by natural law, where Darwinian
contingency actually reigns.)
An absence of variation also stymies Darwinian change by eliminating the
raw material or substrate for any selective mechanism. Evolution in non-varying
populations might be treelike and genealogical, but such a process could not be
Darwinian. One would have to imagine some very unearthlike way to generate
change and diversity—for example, random dispersal of initially identical
creatures to varying environments, followed by a Lamarckian or directly inductive
process of heritable environmental stamping upon all members of a population.
Variation without heredity (that is, an absence of correlation between
properties of offspring and parents) also stymies Darwinian causality. Selection
could occur in a single generation. That is, the biggest or the ugliest might
outreproduce all others, or even ruthlessly murder all small and beautiful
conspecifics—but to what evolutionary avail, if the offspring of survivors then
reconstitute all the original variation in original proportions? If variation occurred
without correlation to parental constitution, but with inherent bias in a given
direction—so that even random mortality produced a trend—