Darwin (1859, 1871) clearly distinguished between natural selection and sexual selection as
different kinds of processes operating on different kinds of traits according to different kinds of
evolutionary dynamics.  For him, natural selection improved organisms’ abilities to survive in an
environment that is often hostile and always competitive, while sexual selection honed abilities to
attract and select mates and to produce viable and attractive offspring.  But this critical distinction
between natural and sexual selection was lost with the Modern Synthesis (Dobzhansky, 1937;
Huxley, 1942; Mayr, 1942; Simpson, 1944), when natural selection was redefined as any change in
gene frequencies due to the fitness effects of heritable traits, whether through differential survival
or differential reproduction.  The theory of sexual selection through mate choice had been widely
dismissed after Darwin, and this brute-force redefinition of natural selection to encompass virtually
all non-random evolutionary processes did nothing to revive interest in mate choice.
     Fisher (1915, 1930) was one of the few biologists of his era to worry about the origins and
effects of mate choice.  He developed a theory of "runaway sexual selection," in which an
evolutionary positive-feedback loop is established (via genetic linkage) between female preferences
for certain male traits, and the male traits themselves.  As a result, both the elaborateness of the
traits and the extremity of the preferences could increase at an exponential rate.  Fisher’s model
could account for the wildly exaggerated male traits seen in many species, such as the peacock’s
plumage, but it did not explain the evolutionary origins of female preferences themselves, and was
not stated in formal genetic terms.  Huxley (1938) criticized Fisher’s model in a hostile and
confused review of sexual selection theory, which kept Darwin’s theory of mate choice in limbo for
decades to come.
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