646 THE STRUCTURE OF EVOLUTIONARY THEORY
thousands of years, tens of thousands, hundreds of thousands. In any given
period of geological time, the number of selective species extinctions that
can have taken place is many orders of magnitude less than the number of
selective allele replacements that can have taken place ... We shall have to
make a quantitative judgment taking into account the vastly greater cycle
time between replicator deaths in the species selection case than in the gene
selection case.I strongly support Dawkins's last statement, but will argue (see pages 703-
712) that, when we factor punctuated equilibria into the equation, species selection
emerges as a powerful force in macroevolution (though not as an architect of
complex organismic adaptations).
Williams has also supported Fisher's argument about the logic of higher level
selection—even in his gene selectionist manifesto of 1966, where he defends the
possibility, but then denies the actuality: "If a group of adequately stable
populations is available, group selection can theoretically produce biotic
adaptations, for the same reason that genie selection can produce organic
adaptations" (1966, p. 110). In his later book, however, Williams becomes much
more positive about the importance and reality of selection at several hierarchical
levels: "To Darwin and most of his immediate and later followers, the physical
entities of interest for the theory of natural selection were discrete individual
organisms. This restricted range of attention has never been logically defensible"
(1992, p. 38).
The developing literature has added three "classical" arguments against
higher-level selection to supplement Fisher's point that cycle times for species
must be incomparably long relative to the lives of organisms. All these arguments
share the favorable property of accepting a common logic but challenging the
empirical importance of legitimate phenomena—a good substrate for productive
debate in science, in contrast with the confusion about concepts and definitions that
so often reigns. In the rest of this section, I shall summarize the four classical
arguments (Fisher's original plus the three additions); note that they can all be
effectively challenged at the level for which they were devised ("group," or
interdemic, selection); and then demonstrate that none has any strong force, in
principle, against the empirical importance of the still higher level of species
selection.
The classical arguments against efficacy of higher-level selection
The usual arguments against higher-level selection admit that such phenomena
must be possible in principle, but deny any meaningful efficacy on grounds of
rarity and weakness relative to ordinary natural selection upon organisms.
WEAKNESS (BASED ON CYCLE TIME). R. A. Fisher's classical argument: How
could species selection exert any measurable effect upon evolution? Rate and
effect depend upon numbers and timings of births and deaths—to provide a
sufficient population of items for differential sorting. But species endure for
thousands or millions of years, and clades count their