708 THE STRUCTURE OF EVOLUTIONARY THEORY
other cannot be explained by competition between their sympatric populations, but
depends upon a species-level trait of the species's full range—in other words,
species selection of the whole, not of avatars or sympatric subsections. I present in
Figure 8-5 a hypothetical case developed by Robert N. Brandon (personal
communication, 1988, Ohio State meeting).
The three species of a clade live on four adjacent volcanic islands. Species 2
can move readily across small oceanic gaps and inhabits all four islands. Species 1
and 3 have limited mobility and live on only one island each. (Species 2 gains no
necessary advantage of the moment thereby.) The population of Species 1 on
Island A, and of Species 3 on Island C, may each exceed the total number of
organisms in Species 2 on all four islands. In fact, on any individual island, either
Species 1 or Species 3 may always fare better than Species 2. Each island
maintains an active central volcano; when the volcano erupts, all life on the island
dies, but the adjacent islands remain unaffected. One fine day, the volcanoes of
Islands A and C erupt. As a consequence, Species 1 and
8 - 5. A hypothetical example of species selection based on traits that belong to entire species—in
this case the full geographic range—and not to avatars or subpopulations thereof. See text for
details of this verbal case developed by R. N. Brandon. Species 2 survives by virtue of its ability
to spread among islands, even though any other species dominates over species 2 on any island
of joint occurrence.