Species as Individuals in the Hierarchical Theory of Selection 733
lower-level processes.
Wright's Rule represents a strong test for putative species selection, but I now
realize that its failure does not eliminate species selection from consideration.
When Wright's Rule holds, a trend must be attributed to species sorting, for no
directional component exists at the lower level of variation among units of sorting.
But if Wright's Rule fails in any particular case, then species selection cannot forge
the trend exclusively—although species selection may still operate as one
contribution in a hierarchical system. A speciational drive may act synergistically
with species selection to intensify a trend. (Since drives tend to be more potent
than selection, a powerful drive, with strong violation of Wright's Rule, will
probably relegate species selection to an insignificant role. But small departures
from Wright's Rule permit a substantial intensification of the trend by species
selection. In any case, and in situations of unusually complete paleontological data,
we should be able to measure the relative strengths of drive and sorting when the
two modes act synergistically.)
Wright's Rule has been tested in some cases, but not often enough—and the
subject remains ripe for future research, including several Ph.D. theses! Gould and
Eldredge (1977) found Wright's Rule validated for Gingerich's data on early
Tertiary Hyopsodus. MacFadden (1986) failed to confirm Wright's Rule in the
evolution of horses, where a directional bias exists for descendant species to arise
at larger body sizes than their ancestors. Arnold, Kelly, and Parker (1995)
validated Wright's Rule for a remarkably complete data set of 342 ancestral-
descendant pairs in Cenozoic planktonic foraminifera. An equal number of species
arose at larger and at smaller sizes than ancestors; see Figure 8 - 7. In a pioneering
study, notable for completeness and density of data (and a consequent capacity to
distinguish among all the various modes of evolutionary change), Wagner (1996)
documented three general and speciational trends in the evolution of gastropods
during the lower Paleozoic (Cambrian through Silurian): towards higher spires,
more inclined apertures and narrower sinuses. For 276 ancestor-descendant pairs
over the entire clade, Wagner confirmed Wright's Rule for spire height and
inclination, where as many species differed from ancestors in a direction away
from the general trend, as along the ultimately favored route. But data for sinus
width, where a statistically significant bias exists for speciation in the direction of