724 THE STRUCTURE OF EVOLUTIONARY THEORY
clades could not dominate the history of life, as they manifestly do, particularly in
marine invertebrates (clams, snails, horseshoe crabs, brachiopods, all from the
Paleozoic to the present); nor, among more rapidly changing terrestrial clades,
could dinosaurs (not to mention the more stable insects) persist and rule for so long
in a world where most species evolved continually by the analog of ontogenetic
drive in the Lamarckian mode.
Of course, we could posit other reasons for braking the rapidity and efficiency
of change by ontogenetic drive in macroevolution—disruption of trends by mass
extinction; high frequency of trends that benefit organisms but harm species
(peacock's tails), for example. But I suspect that the simplest of all reasons will
explain the evident pattern: the species-level analog of ontogenetic drive—gradual
transformation within a species—just doesn't occur very often.
Finally, I note that R. A. Fisher's classic argument for the impotence of spe-
cies selection rests on the standard assumption that this mode of driving does
prevail in evolution. For, if most species, most of the time, changed gradually from
within (see pp. 644-646), then selection among species would be, as Fisher rightly
noted, an operative but impotent process, capable of generating only an
insignificant amount of change relative to the dominant and ubiquitous drives of
anagenesis. But if anagenesis rarely occurs, Fisher's argument collapses. I wonder
if Fisher ever explicitly realized that anagenesis would trump species-selection
because anagensis is Lamarckian at the species level, while species selection is
Darwinian at the same level—for Lamarckian processes can always overwhelm the
much weaker force of Darwinian change if both operate generally and in an
unimpeded manner at the same level.
Reproductive drive: directional speciation as an important
and irreducible macroevolutionary mode separate from
species selection
Thus, the first category of ontogenetic drive illustrated interesting differences in
style between levels, but little variation in effect—for I conclude that this mode has
scant impact upon evolution at either level. But when we consider the second
category of reproductive drive (biased production of offspring that vary in a given
direction from parents), we encounter one of the chart's most striking disparities—
a crucial, yet almost entirely unrecognized and unexplored difference in basic
pattern between micro- and macroevolution. To choose a hypothetical example of
simplified form but maximal clarity: Suppose that each collectivity (a population
for the organismal level, or a clade for the species level) contains ten individuals
(organisms or species, for the two levels). Each individual gives birth to a single
offspring, and all offspring have identical life spans and reproductive capacities.
Thus, no selection at all
"capable of turning sparrow-size into ostrich-size bones, and back again, about 54 times."
Clearly, anagenesis at virtually any rate high enough to stand out above measurement
error over a human lifetime cannot be sustained in a unidirectional manner for
meaningful intervals in geological time.