604 THE STRUCTURE OF EVOLUTIONARY THEORY
"hold together" long and discretely enough so that the differential proliferation and
survival of some denies vs. others can propel the general increase of an "altruistic"
allele (promoting demic success), even while the allele's frequency declines within
groups as "selfish" alternatives prevail in conventional natural selection among
bodies. If demes can "hold together" by this operational criterion of evolutionary
outcomes, then they possess "sufficient stability" to be regarded as individuals on
functional grounds within selectionist theory.
Traditionally, biologists have not been willing to imbue species with these
requisite criteria for individuality. Species, in an argument dating to both Lamarck
and Darwin, have often been construed as mere names of convenience attached to
segments of evolving continua without clear borders. Under this gradualistic and
anagenetic view (see Fig. 8-1), a species near the end of its arbitrary existence
must be phenotypically more similar to a forthcoming descendant than to the initial
ancestor. (Indeed, under strict gradualism, we even face the definitional absurdity
that the last generation of an ancestor should be reproductively isolated from its
own offspring—that is, the first generation of a new descendant. Some creatures
may eschew such incest on moral or adaptive grounds, but no one would gainsay
the biological possibility.) Thus, on this traditional view, species cannot maintain
sufficient temporal stability to be called individuals. In addition, species do not
have discrete birth points if they branch from their ancestors at rates no different
from characteristic tempos of transformation during their subsequent anagenetic
lifetimes (see Fig. 8-1). At most, some species display clear termination in
extinction (but others evolve gradually to descendants.) Thus, species do not
function as good vernacular individuals if gradualism and anagenesis pervade the
history of life.
Even so—or as long as most species arise by splitting of lineages rather