Species as Individuals in the Hierarchical Theory of Selection 721
birth itself (speciation)—and equated the correlate at one level with the
phenomenon at the other. The proper analog of mutation, a source of variation for
new individuals, is the change that insures reproductive isolation between species
(with geographic isolation as a usual precondition, and drift and selection as
mechanisms)—see line 115.
This difference underlies the two important disparities listed as lines II4a and
II5a—one favoring the evolutionary capacity of organisms, the other of species.
Sexual organisms can spread favorable variation to other individuals in the
collectivity by recombination. But the favorable features of new species remain
stuck in the species and its lineal descendants, and cannot be spread to other
species in the clade—except in the infrequent circumstance of hybridization among
species in a clade of multicellular forms. (By contrast, lateral transfer seems to be
common in the evolution of prokaryotic lineages.) This preclusion of lateral spread
puts a strong damper upon evolution within clades. The same limitation, of course,
affects asexual vs. sexual organisms— and represents a standard argument for the
great advantage of sex, and its evolutionary prevalence, in complex metazoans
(Williams, 1975; Maynard Smith, 1978).
Species do gain advantages, on the other hand, in the necessary association of
birth with change (sometimes small in extent because reproductive isolation can
develop with minimal genetic change, but usually quite substantial). This input
helps to offset the disadvantages of small population sizes (species in clades) for
species selection. The asexual budding of a new species always yields novelty; the
asexual budding of a new organism usually yields clonal identity, and only
produces novelty if mutation intervenes.
Contrasting Modalities of Change: The Basic Categories
The greatest interest in this analogy lies in the third category of contrasting
modalities. Since individuals vary and collectivities evolve (by cumulative changes
in their contained individuals) in the standard formulation, I shall first define the
three major styles of change within collectivities (populations for the organismal
level, and clades for the species level).
DRIVE. This term has often been used for particular cases—meiotic drive, or
molecular drive, for example—but deserves to be formalized and generalized. A
driving process transforms a collectivity by directionally changing its contained
individuals from within. Drives should be construed as opposite or at least
orthogonal to selection. Drives produce change by directional transformation of
relevant individuals, not by differential proliferation of some kinds of individuals
over others. Thus, in pure cases of drive, change occurs without any differential
proliferation. In the paradigm cases of drive, either an individual alters in the
course of ontogeny and passes these modifications to offspring, while all
individuals produce the same number of offspring with the same reproductive
capacities (so no selection can take place); or an individual produces offspring
endowed with directional differences from its own constitution—but again, no
differential reproduction occurs, and no selection can take place. (As one
complexity—an ineluctable consequence of the hierarchical