728 THE STRUCTURE OF EVOLUTIONARY THEORY
macroevolutionary theory extends well beyond the phenomenology of Darwinian
analogs in species selection.
As an additional argument for the importance of directional speciation as a
driving force in evolution—and as an example of interesting complexity
engendered by the hierarchical model (and of differences in the character of
explanation between this hierarchical reformulation, and the traditional one-level
world of Darwinian evolution)—note what often happens when causes at one level
correlate with emergent properties involved in causes at a higher level; for we then
encounter the fascinating situation of disparate theoretical meanings for inexorably
linked phenomena at two levels. (We have already discussed one common example
in the causation of higher-level drives by lower-level selection.) Another important
example, potentially encompassing one of the dominant phenomena of
macroevolution, translates the results of ordinary selection at the organismal level
into strong constraints acting as causes of directional speciation at the species
level. In this sense, when considered at the appropriate higher level,
macroevolutionary pattern results much more from immediate constraint, and less
from the traditional selectionist mode, than we have generally been willing to
allow—thus suggesting another potentially important reform and expansion of
Darwinian thinking (see Chapter 10 for a fuller discussion).
Consider two cases of cladal trends produced by the driving cause of
directional speciation. Figure 8-6 depicts the common pattern in both examples. At
a starting point, the clade contains two kinds of species in equal numbers—those
bearing trait A, and those bearing trait B. Every reproducing species generates two
daughters and no variation exists for differences in species birth rates among those
species that have offspring—so no species selection can occur. Evolution proceeds
rapidly by directional speciation because A-Species can only produce A-
Daughters, while B-Species produce 50 percent
8 - 6. A cladal trend produced entirely by directional speciation with no species selection. A
species can only produce A daughters, while B species produce 50% A daughters and 50% B
daughters. Under these conditions of strongly directional speciation, a powerful trend towards A
leading to quick disappearance of B from the clade, will arise, even under a regime of random
mortality among species.