Species as Individuals in the Hierarchical Theory of Selection 731
on mammalian vertebral columns; McShea, Hall, Grimsson and Gingerich, 1995,
on mammalian teeth; and Boyajian and Lutz, 1992, on ammonite sutures).
with Directional Speciation Species Selection, Wright's Rule, and the Power of Interaction
with directional speciation
I have long regarded species selection as the most challenging and interesting of
macroevolutionary phenomena, and the most promising centerpiece for
macroevolutionary theory. While I continue to espouse this view, my rethinking
for this chapter has led me to appreciate the significant power of two other species-
level processes: drives of directional speciation as just discussed (see also Gould,
1982c), and species drift, the higher-level analog of genetic drift. I would now
argue that the interaction of these three processes sets the distinctive character of
macroevolution.
As for natural selection at the organismic level, the two major modes of
species selection operate by differential rates of generating daughter species (the
analog of birth biases in natural selection) and differential geological longevity
before extinction (the analog of death biases in natural selection). At the species
level, however, the difference between these two modes does not rest upon the
same basis that distinguishes their analogs at the organismic level.
At the organismal level, natural selection by birth bias works mainly upon
such "internal" traits as reproductive rate and brood size, and often doesn't increase
adaptation in the conventional sense of phenotypic molding to better
biomechanical design for local environments. For example, an organism gains a
large selective advantage merely by breeding a bit earlier, though nothing else
about the phenotype need alter (Gould and Lewontin, 1979, referred to this mode
as "selection without adaptation"). But natural selection by death bias among
organisms usually yields phenotypic adaptation for better fit to the ambient
environment.
At the species level, however, our main concern moves to an interesting
difference in causal locus. Most cases of selection by differential speciation
operate by the interaction of an irreducible species-level character—some feature
of population structure—with the environment, and therefore represent genuine
species selection. After all, and as stated before, organisms don't speciate; only
populations do. But for selection by differential extinction, a higher frequency of
cases can probably be explained as the simple summation of organismal deaths,
and may therefore be causally rendered at this conventional lower level—for both
organisms and species die. Thus, students of species selection have rightly
focussed on differential speciation as their most promising category (see Gilinsky,
1981, for both theoretical arguments and empirical examples).
However, the most interesting of all differences between organismal and
species selection may lie not in the phenomena themselves, but rather in the
character of their interaction with the two other primary modes of evolutionary
change: drives, and drift (I shall discuss drift in the next section). Our