Species as Individuals in the Hierarchical Theory of Selection 737
When we move to the second category of random results achieved by sorting
in the colonization of new places—the analog of the founder effect—then
comparison with the organismal version becomes less straightforward, although we
may be confident that the species-level version holds potential for great importance
in evolution. The species-level analog, which I will call "founder drift" (see lines
IIIC2 and IIIC2a), does not work through a simple phenotypic difference between
a colonizing species and the parental stay-at-home—for all species differ by
definition, and disparities arise by the usual combination of selective and random
effects, usually expressed at the organismic level. The stochastic analog to Mayr's
"founder effect" at the organismal level lies in random aspects of the differential
capacity for proliferation of new species in allopatric regions of a clade's full
range.
A hypothetical example will illustrate this unfamiliar concept. Suppose that a
clade contains only two species, living in adjacent islands with similar
environments. The islands, however, lie on different oceanic plates, and
movements of plate tectonics cause the coalescence of one island with a large
neighboring continent, while leaving the other island in the midst of the ocean. The
species on the continent proliferates into a large subclade of new species, while the
species on the island, lacking any room for expansion, remains as the only species
of its subclade. Because the process of speciation yields phenotypic disparity
intrinsically, the founding continental species will differ from its insular sister
species. Therefore, the clade will show a strong trend in the direction of
autapomorphic traits possessed by the continental founder. But such a trend will
often be entirely random with respect to the plurified traits of the continental
founder. That is, these spreading traits may be completely neutral in the crucial
sense that if the other (insular) species had colonized the island that coalesced with
the continent instead, its autapomorphies would have proliferated, and the cladal
trend would have proceeded with the same force, but in the opposite direction.
Only the luck of residence on one island rather than the other (and not any
preferential interaction of some traits vs. others with the environment) leads to the
differential proliferation of one species's traits over those of the sister species.
Situations of this sort must be common, if not virtually canonical, in
evolution. Almost any two geographic regions must maintain differential capacity
to house species of a given clade. If both regions are colonized by founding
species, and, many million years later, one region holds substantially more species
than the other, the random component of spatial and ecological opportunity must
often play a greater role in differential speciation than the selective force of greater
capacity for differential proliferation in one subclade vs. the other based on
interactions of traits with environments. I use the term random in a special, but
surely legitimate, sense. Suppose that a large and ecologically diverse Region 1
can accommodate 50 species of a subclade, while smaller and more homogeneous
Region 2 can only maintain 10 (I realize that species create their own
environments, and that regions don't maintain fixed numbers of available
addresses, but I invoke this simplification for