608 THE STRUCTURE OF EVOLUTIONARY THEORY
considerably, though not to a one-dimensional Euclidean line. But, of course, the
replication of organisms does not happen instantaneously either. If absolutely
discrete boundaries are required for individuals, then there are no individuals in
nature. It is only our relative size and duration which make the boundaries between
organisms look so much sharper than those between species."
But, to continue the Euclidean metaphor, and using an appropriate ruler with
(say) a minimally noticeable geological increment equal to 10,000 years, the
boundaries of many species do become momentary under punctuated equilibrium.
Stasis persists for a long run of increments. At a commonly observed duration of 5
to 10 million years for marine invertebrate species in the fossil record (Raup, 1985;
Stanley, 1985), one thousand increments of stasis would represent the geometry of
a species lifetime, while even a million for the much shorter average duration of
terrestrial mammalian species yields 100 increments. By comparison, many
(probably most) events of speciation unroll within a single increment—leading to
abrupt and momentary origin at geological scales, and the right-angle convention
that has become standard for plotting the emergence of species under punctuated
equilibrium (see Fig. 8-3).
Criteria for evolutionary individuality
The vernacular criteria discussed above provide necessary, but obviously
insufficient, conditions for identifying an entity as an evolutionary "individual"
with the capacity to act as a causal agent in a process of Darwinian selection. Most
unambiguous vernacular individuals cannot operate as Darwinian actors. The earth,
for example, surely merits designation as a well-defined individual—with a
specifiable birth (perhaps attended by some initial fuzziness as a primordial
fireball), sufficient stability over billions of years (including enough climatic
homeostasis to provide a stage for the history of life), and a forthcoming rapid
death (presumably by absorption after the sun burns out some five billion years
from now, and expands in diameter at least to the orbit of Jupiter). But the earth
remains "infertile" in the crucial Darwinian sense of reproductive potentiality.
Planets do not have children, and therefore cannot function as Darwinian
individuals.
I do not cite this example to win an argument by ridicule, but rather to
emphasize, once again, that all definitions must be embedded within theories. Mere
vernacular individuality does not suffice for identification as a causal actor in
Darwinian theory. Evolutionary individuality (or, more strictly, Darwinian
individuality, for different theories of biological change may entail other criteria)
requires an additional set of attributes rooted in two features of Darwin's world: the
genealogical basis of evolution as a branching tree, and the causal efficacy of
selection as the leading process of evolutionary change.
REPRODUCTION. Darwinian individuals must be able to bear children.
Biological evolution is defined as a genealogical process. Darwinian evolution
operates by the differential increase of your progeny (or whatever you pass