800 THE STRUCTURE OF EVOLUTIONARY THEORY
microevolution within populations. (Gene selectionists make a crucial error in
arguing that sexual organisms are not stable enough to be regarded as units of
selection because they must disaggregate in forming the next generation. But units
of selection are interactors, and the "sufficient stability" required by the theory
only demands persistence through one episode (generational at this level) of
selective interaction to bias reproductive success—as organisms do in the classical
Darwinian "struggle for existence," see full discussion on pages 619-625.)
Organisms achieve this stability through ordinary mechanisms of bodily coherence
(a protective skin, functional integration of parts, a regulated developmental
program, etc.).
What, then, produces a corresponding stability for units of macroevolution?
Species-individuals are constructed as complex units, composed of numerous local
populations, each potentially separate (at any moment) due to limited gene flow,
and each capable of adaptation to unique and immediate environments. Thus, in
principle, substantial evolution can occur in any local population at any time
during the geological trajectory of a species. A large and developing literature,
much beloved by popular sources (media and textbooks) for illustrating the
efficacy of evolution in the flesh of immediacy (that is, within a time frame
viscerally understood by human beings), has documented these rapid and adaptive
changes in isolated local populations—substantial evolution of body size in
guppies (Reznick et al., 1997), or of leg length in anolid lizards (Losos et al.,
1997), for example (see Gould, 1997f).
But these changes in local populations cannot gain any sustained
macroevolutionary expression unless they become "locked up" in a Darwinian
individual with sufficient stability to act as a unit of selection in geological time.
Local populations—as a primary feature of their definition—do not maintain such
coherence. They can in principle—and do, in the fullness of geological time,
almost invariably in practice—interbreed with other local populations of their
species. The distinctively evolved adaptations of local populations must therefore
be ephemeral in geological terms, unless these features can be stabilized by
individuation—that is, by protection against amalgamation with other Darwinian
individuals. Speciation—as the core of its macroevolutionary meaning—provides
such individuation by "locking up" evolved changes in reproductively isolated
populations that can, thereafter, no longer amalgamate with others. The Darwinian
individuation of organisms occurs by bodily coherence for structural and
functional reasons. The Darwinian individuation of species occurs by reproductive
coherence among parts (organisms), and by prevention of intermingling between
these parts and the parts of other macroevolutionary individuals (that is, organisms
of other species).
Rapid evolution in local population of guppies and anoles illustrates a
fascinating phenomenon that teaches us many important lessons about the general
process of evolution. But such changes can only be ephemeral unless they then
become stabilized in coherent higher-level Darwinian individuals with sufficient
stability to participate in macroevolutionary selection. These local populations
usually strut and fret their short hour on the geological stage, and then disappear by