Seeds of Hierarchy 241
or breaking point, for extrapolationist theories that seek the causes of
macroevolution in microevolutionary processes centered upon organismic
selection. Darwin understood and accepted this challenge; his principle of
divergence marks his attempt to depict trends as extrapolated results of natural
selection. (The principle of divergence attempts to explain morphological trends by
specialization and progressive departure from ancestral form, and also to account
for numerical trends by multiplication of some taxa at the expense of others within
a clade.)
Advocates of species selection hold that trends must be described as the
differential birth and death of species (not the simple anagenetic extrapolation of
change within a population), and that the causes for such differentials must be
sought, at least in part, in irreducible species-level fitness (see Chapter 8). The
standard extrapolationist rejoinder invokes two arguments: (1) Differential death
and survival rather than differential birth (of species) usually fuels trends. The
death and persistence of groups can be reduced more easily to organismic
competition, while differential production of species more often demands
irreducible causes, for an organism cannot speciate by itself, while the death of a
population may represent no more than the accumulated demise of all organisms.
(2) The cause of differential survival or death must be reducible to ordinary natural
selection.
Darwin did not offer his principle of divergence as a rejoinder to any
explicitly developed theory of species selection, for no such formulation existed
when he wrote. But he understood the logical requirements of his theory so well
that he provided the necessary rationale without the spur of a formally stated
alternative. He also, and uniquely, reinforced his argument with an illustration of
the need for differential survival of certain kinds of variants within random arrays.
In Natural Selection, Darwin presents his case as a second figure (reproduced here
as Fig. 3-6) that he did not include in the abridged Origin of Species in 1859.
(Virtually no one knew about the existence of this figure or argument until Stauffer
published the manuscript of Natural Selection in 1975.)
The basic figure of both Natural Selection and the Origin illustrates Darwin's
claim that only a few vigorous species will produce the variants leading to the
"recruitment" of new species. (These vigorous species are the extreme forms
favored by ordinary natural selection—A and M in Natural Selection, A and I in
the Origin, see Figs. 3-5 and 3-6.) The variants of these vigorous species radiate in
an even fan, or random array, about the modal form of their ancestor. A trend then
arises because ordinary natural selection favors extremes within the fan. Darwin
recognizes that a trend to specialization and diversity cannot be generated only by
the greater vigor of extreme species in the initial array; he must also defend a
second proposition about differential survival among offspring of these favored
extremes.
The second figure of Natural Selection now comes into play (Fig. 3-6).
Darwin shows us what happens under a regime of random survival within the fan
of variants generated by the favored vigorous species at ecological extremes, as
opposed to a regime of selection positively directed towards extreme members