726 THE STRUCTURE OF EVOLUTIONARY THEORY
We may postulate any number of plausible circumstances that would generate
directional biases in the origin of new species—thus producing a cladal trend
without any contribution from species selection. Moreover, potential causes for
directional bias exist at all levels—organismic, demic, or species— thus greatly
expanding the scope of the phenomenon. As a central theoretical point, directional
speciation, when based on irreducible species-level properties, represents a style of
independent and causal macroevolution not based on species selection. Thus, the
claim for an independent body of macroevolutionary theory does not depend upon
the validity and high relative frequency of the Darwinian analog most often
discussed as a paradigm case, namely species selection. Directional speciation,
when based on irreducible species-level traits or processes, designates another
category of intrinsically macroevolutionary change.
To continue in the hypothetical mode with the example cited previously, one
can easily imagine how a cladal trend, attributable entirely to reproductive drive
(and not at all to selection), and leading to decreasing average (organismal) body
size, might be caused at either the organismal, demic, or species level. At the
conventional organismic level, a pervasive environmental change over the entire
region of a clade's occupancy might favor natural selection for smaller bodies.
(Perhaps, to choose a somewhat cardboard example, a temperate region has
become tropical, and smaller organisms now gain advantages within each species
of a clade by the adaptive correlates of Bergmann's Rule.) Each species produces a
single daughter species and then dies out—so no selection can occur at the species
level. But if most species, in the new climatic regime, originate at smaller average
body size because natural selection favors this trait among the organisms in each
species, then the cladal trend arises by directional speciation with a cause based on
selection at the organismal level—the classic case of a drive at a higher level
produced by directional selection among contained parts at a lower level.
For a hypothetical case based on interdemic selection, suppose that each
species in a clade develops ten small and isolated demes at the periphery of the
parental range. Suppose that average body size in these peripheral isolates varies
randomly around the parental mean. Suppose further that, for each species, only
one of the ten peripheral demes survives, intensifies its differences, and eventually
becomes a new species—while the parent and the other nine peripheral isolates all
die. Again, no species selection can take place, for
rates intrinsically from the parent; how then, may this offspring be kept sufficiently like the
parent to preserve the collectivity of the population? An opposite problem attends the birth of
species. At the species level, new individuals are born by speciation, which enhances change.
But species do not separate intrinsically from their parents. They are born in fuzzy
continuity. Their separation may be difficult. They must be cast out, or they will reintegrate.
Necessary change at speciation enhances this defining process of casting out from the parent.
The newly born species faces a structural problem opposite from the neonatal organism's
dilemma: how may the new species-individual become sufficiently unlike the parent to be
cast out, thus enhancing the collectivity of the clade by adding another part? In short, the
new metazoan organism forms outside the parent: how can it be kept close? The new species
separates with difficulty from the parent: how can it be cast out?