738 THE STRUCTURE OF EVOLUTIONARY THEORY
the sake of argument). Subclade A invades Region 1, while Subclade B begins in
Region 2. The resulting strong cladal trend toward the autapomorphic characters of
Subclade A cannot be called accidental in the global sense—for Region 1 does
predictably accommodate more species. But the trend may be accidental in the
sense that Subclade A, rather than Subclade B, happened to invade the more
prosperous region—and that if Subclade B had been the colonizer, its progeny
would have done equally well, and would have dominated the cladal trend with the
same force actually shown by Subclade A. In this case, we call the trend random
because A's success does not arise from any superiority of an interacting trait (vs.
B's phenotype), but only from the accident of colonizing a more propitious place
(see Eble, 1999, and Chapter 11 of this book for a discussion of this evolutionary
meaning of "random").
As with the relationship between directional speciation and species selection,
these two forms of species-level drift must often interact with the other main cause
of sorting—i.e., selection—to produce a trend (as when Subclade A, in the
example just above, increases both by the good fortune of greater opportunity, and
by selective benefits conferred by its traits). The organismic level may experience
a higher relative frequency of domination by selective forces, but the world of
species evolves by complex interactions among the processes of drive, selection,
and drift.
The Scaling of External and Internal Environments
I have not tried to develop an exhaustive comparison between levels for influences
of external and internal environments upon the modes of change discussed in
previous sections. But I offer a few sketchy comments to encourage further work
in this area.
For environmental factors that induce competition among individuals and
therefore establish selection pressures (line IVA of the chart), I contrast modes that
involve direct contact among individuals with those that can proceed in allopatry.
At the organismic level, this contrast exposes a strong correlation between
prevalence of biotic factors in direct contact and abiotic factors in allopatry. At the
species level, a different correlation may dominate: the association of selection by
differential elimination with direct contact, and selection by differential birth with
allopatry (lines IVA1 and IVA2).
This contrast also leads to different implications at the two levels. At the
organismic level, as Darwin himself argued in his primary justification for progress
in the history of life (see Chapter 6), the biotic mode correlates more often with
adaptation by general biomechanical improvement, and the abiotic mode with
adaptation to local circumstances of the physical environment, with no vectorial
component as environments fluctuate randomly through time. At the species level,
we may expect to find a strong correlation of selection by differential elimination
with potential reduction to the organismal level, while selection by differential
birth represents the most promising domain for true and irreducible species
selection.