in complexification)." Darwin developed the first testable and operational theory of
evolution by locating all causality in the palpable mechanism of natural selection.
- In the first generation of Darwinian debate, August Weismann, clearly the
most brilliant theorist of his time, and the only biologist (besides Darwin) who
fully grasped the logic and implications of selection, wrestled with levels of
selection throughout his career, and along an interesting path, finally developing a
full hierarchical theory that he explicitly identified as the most important
conclusion of his later work. He began by trying to refute Lamarckian inheritance
(and Herbert Spencer's vigorous defense thereof) by advocating the Allmacht
(omnipotence, or literally "all might" or complete sufficiency) of natural selection.
He first attributed the degeneration of previously useful structures (a bigger
problem for Darwinism than the explanation of adaptive features) to what he called
"panmixia" (not the modern meaning of the term, but the effect of recombination,
in sexual reproduction, between adaptive elements and inadaptive elements no
longer subject to negative selection); then realized that this process could not
explain complete elimination, thus leading him to propose a lower level of
subcellular selection, potentially acting in opposition to organismal selection, and
called "germinal selection"; and finally recognized that if levels of selection
existed below the organismal, then the same logic implies the existence and
potency of supraorganismal levels as well. - Darwin himself provides the best 19th century example—previously
unrecognized because Darwin omitted this material, originally written for the
unpublished "long version," from the Origin—of the need for a hierarchical theory
of selection in any full account of the phenomenology of evolution. Entirely
consistent single-level theories cannot be carried through to completion. Darwin
admitted important components of species selection in capping his (still
unsatisfactory) explanation for an issue that he ranked second in importance only
to explaining the anagenesis of populations by natural selection: the resolution of
organic variety and plenitude by a "principle of divergence" (his terminology). I
document the largely unrecognized emphasis that he placed upon this principle of
divergence (for example, the Origin's famous single figure does not illustrate
natural selection, as generally misinterpreted, but rather the principle of
divergence). Darwin struggled to explain this descriptively higher-level
phenomenon of taxonomic diversification as a fully predictable consequence of
ordinary organismal selection, but he could not proceed beyond an argument that
he himself finally recognized as forced, and even a bit hokey: the claim that natural
selection will always favor extreme variants at the tails of a distribution for a local
population in a particular ecology (the Origin's diagram represents an
exemplification of this claim). Eventually, Darwin realized that he needed to
invoke species selection for a fell explanation of the success of speciose clades—
and this unknown argument, rather than his well-documented defense of group
selection for human altruism, represents Darwin's most generalized invocation of
selection at supraorganismal levels.
64 THE STRUCTURE OF EVOLUTIONARY THEORY