Species as Individuals in the Hierarchical Theory of Selection 727
each parent spawns one and only one daughter. (I realize, of course, that these
strictures sound absurd if construed as actual and coordinated occurrences in
nature; I am only following the time-honored heuristic method in science of
constructing "pure" end-member hypothetical to help clarify our thoughts.)
Finally, let us say that, in general, the surviving (and ultimately speciating)
deme lies at or near the smaller-bodied end of the random distribution (around the
parental mean) of average body size. Let us also posit that the smaller average
body size of new species arises as a consequence of a deme-level property
conveying differential success in interdemic selection among the ten peripheral
isolates initially spun off from each species. An obvious (and not implausible)
reason might be found in a strong correlation between small bodies and larger N in
any population (the "more ants than elephants" principle, albeit in a more restricted
range). The surviving deme might owe its success to generally larger population
size in a tough peripheral environment.
The cladal trend to smaller body size among species would then arise by a
drive of directional speciation (new species biased to originate with smaller-bodied
organisms than those of their ancestors, in a situation where no species selection
can occur). The cause of the trend, in this hypothetical case, will be interdemic
selection—for the ten peripheral isolates arise as demes of the parental species.
Selection among these demes favors those with smaller average body size, based
on correlation with the causally controlling deme-level property of larger
population size. This deme-level property confers an irreducible fitness upon
demes in their interaction with the environment. (In an extreme, albeit improbable,
case, interdemic selection based on larger population size could even outweigh
negative organismic selection against small bodies.) Again, a drive at the species
level arises by selection among lower-level parts, in this case demes rather than
organisms.
Finally, an irreducible species-level character may cause a drive at the species
level. Suppose that each species spins off only one peripherally isolated population
and that, invariably, the parental population dies while the peripheral isolate
becomes a new species. Suppose that the single peripheral deme, generated by
each species, generally features organisms with a smaller average body size than
the organisms of the parental population. Suppose that this directional bias arises
as a result of a species-level trait in the parental population. Perhaps, for example,
the social structuring or territorial system of the parental population preferentially
excludes smaller organisms of both sexes, and that these smaller organisms
therefore tend to migrate to the species border, where they aggregate to form the
isolated population that will generate a new species. (Again, the case merely
requires conceivability for purposes of illustration, not plausibility.) In this
situation, no selection can occur at the species level because each parental species
produces one daughter species and then dies. The cladal trend to species with
smaller average body size arises by the driving process of directional speciation—
and the cause lies in a species-level trait of the parental population. As stated
above, we here encounter a case of irreducible macroevolution not based on
species selection. Examples of this kind illustrate that the domain of independent