Species as Individuals in the Hierarchical Theory of Selection 725
can take place. Now suppose that a strong bias exists in production of offspring, so
that 80 percent arise with smaller bodies than their parent (lower weight for the
offspring organisms, lower average body weight for the offspring species).
Suppose also that this pattern continues from generation to generation. This driving
process would generate a strong trend to smaller bodies in the collectivities at both
levels—a gradual trend to decreased body size in the population at the organismal
level; and to species with smaller average body sizes within the clade at the species
level.
As discussed previously (p. 691), reproductive drives of this kind can occur at
the organismal level, and a variety of names for such processes exist, including
mutation pressure and meiotic drive. But the Darwinian tradition has always
regarded such phenomena as insignificant as a consequence of their rarity. Indeed,
these processes must be rare in a fully Darwinian world, because reproductive
drives violate the necessary precondition of undirected variability for natural
selection (see pp. 144-146). Darwinians did not win this debate by simple logic or
evident factuality, but only by a great intellectual struggle marking a crucial
episode in the history of evolutionary thought. The classical debate about
orthogenesis, for example (see Chapter 5), centered upon the Darwinian denial of
such reproductive drives, which, as the competing orthogeneticists all realized,
would overwhelm selection by higher efficacy—if they existed. Perhaps such
reproductive drives rarely occur at this level in nature because, having no known
basis for inherent adaptivity, they have been actively suppressed by organismal
selection—another potential example of the most distinctive feature of organismic
individuality: the power evolved by functional integrity to suppress lower-level
selection from within.
However, when we move to the species level, the analogous driving
phenomenon of directional speciation suffers no constraint or suppression—and
may represent one of the most common modes of macroevolution. Two major
reasons underlie the high potential frequency for directional speciation (as opposed
to the rarity of its analog at the organismal level—see line III2a on the chart). First,
as noted in several other contexts, the species-individual does not maintain
integrity (as the organism does) by suppressing differential proliferation of some
parts over others. Since drives at an upper level arise by differential proliferation of
lower-level units, this absence of suppression leaves a large open field for driving
processes to operate at the birth of new species. Second, since new species-
individuals must arise with sufficient heritable novelty to win reproductive
isolation from their parent (whereas children of asexual organisms may be clonally
identical with parents), all species births include genetic change as an automatic
consequence. Any statistical directionality in such changes among species in a
clade will produce a trend by drive. *
*At the risk of an unwarranted metaphorical excursion into anthropomorphic imagery,
one might contrast limited change at organismal birth with necessary change at species birth
in the following manner: New metazoan organisms arise by a process of complex de-
velopment, which must discourage change for reasons recognized ever since von Baer
formulated his laws of embryology (1828). At the organismal level, the new individual sepa-