Species as Individuals in the Hierarchical Theory of Selection 735
narrower sinuses, falsified Wright's Rule (see Fig. 8-8) and documented a drive of
directional speciation. Wagner further demonstrated (1996, p. 1000) that "this bias
is distributed throughout the entire clade," for three major subclades all display the
drive. Nonetheless—and showing the power of such data to identify and tease apart
the different components of a trend into their relative quantitative strengths—
Wagner also documented a component of species sorting in the overall trend to
narrower sinuses, for "species with wide sinuses were significantly less likely to
survive the end-Ordovician mass extinction" (1996, p. 990).
I would go further and suggest that synergisms of drive and sorting (as
Wagner has documented for the trend to narrower sinuses in Paleozoic gastropods)
should be common in the history of many clades, and probably mark a powerful
mode of macroevolution distinct from conventional microevolution, where such
synergism must be rare. Good a priori reasons exist for supposing that features
biasing the directionality of speciation might also favor sorting towards the same
end. Such synergism should be most evident when the causes of both bias and
sorting work at the same (usually organismic) level—as when, for example, a trait
under strongly positive organismic selection (like large body size) arises
preferentially in speciation events, and then promotes the greater longevity of
species so originating. But such synergisms may also be common when causes
differ in level—as when, for example, a drive occurs by organismal selection, and
species-selection then causes sorting in the same direction. For, unlike the situation
at the next lower pairing of levels (where genie and cell lineage selection so often
run counter to the interest of organismal selection, and consequently become
suppressed), selection at the organismal level does not conflict in principle with
selection at the species level. Selection at these two levels should, therefore, be
synergistic as often as opposed. Such synergisms should therefore be frequent and
powerful in macroevolution.
Phenomena in Microevolution Species Level Drifts as More Powerful than the Analogous
phenomena in microevolution
At the organismal level, the second major mode of sorting—drift by random
processes—operates in two ways that should be distinguished both for potentially
different roles and frequencies at this level, and because the species-level analogs
diverge even more clearly. We may distinguish random shift within the
collectivity—called genetic drift at the conventional organismal level— from
random effects introduced at the founding of new demes or species by small
numbers of organisms. Mayr (1942) introduced the term "founder effect" to
distinguish this second category (though the basic mechanism does not differ from
ordinary genetic drift), and to emphasize that the differences initiating a new
species need not arise entirely by natural selection, but may be significantly
enhanced by random effects at the outset, because a small number of founders will,
for stochastic reasons, surely not begin a new population with the same gene
frequencies as the ancestral population, while some