and receiver properties. Linkage disequilibrium is a measure of the nonrandom assortment of alleles at different loci. It is a process that has
generated considerable interest in sexual selection theory (e.g., Andersson
1994), but its generalities can be applied to the more general problem of
signal–receiver evolution. As part of this theory of runaway sexual selection, Fisher (1930) was the first to suggest that linkage disequilibrium plays
an important role in the evolution of sexually selected male signal traits and
female preferences for traits. It has been more recently applied to “good
genes” theories of sexual selection (Pomiankowski 1988; Grafen 1990).
The crux of linkage disequilibrium is that traits that are not under selection can evolve if they are genetically correlated with traits that are under
selection. In the parlance of evolutionary genetics, the trait subject to selection is said to be under direct selection. If another trait variant is more likely
to be associated statistically with the trait under direct selection than
expected by random chance, the two traits are genetically correlated or in
linkage disequilibrium. The second trait, not subject to direct selection, is
under indirect selection by virtue of its genetic correlation with the first trait
and can evolve as a correlated response to the trait under direct selection.
It is the observation that a trait can evolve even if it is not subject to
direct selection that makes linkage disequilibrium especially relevant to
sexual-selection studies. In lekking species of animals, males gather to signal
for mates and provide them with few resources besides sperm. It is assumed
that in many cases, regardless of with whom the females choose to mate,
the number of offspring they birth will be the same. Yet—and this is the
paradox—females sometimes assiduously choose their mates (in many
cases, the signal parameters that influence mate choice are well-known
(Ryan and Keddy-Hector 1992; Andersson 1994)), with only a few males
on the lek gathering a majority of the mating success. How could such
preferences evolve if there is no difference in the reproductive success of
females exerting different preferences or, for that matter, no difference in
the reproductive success of females exhibiting a preference or mating at
random? Two major hypotheses, runaway sexual selection and good genes
selection, have been offered as solutions to this paradox, and both are
dependent on indirect selection and linkage disequilibrium. Even though
this problem of evolution through linkage disequilibrium is usually
addressed within the context of male trait/female preference evolution, in
many cases the male trait is a long-distance signal and the female preference emerges from the interaction of this stimulus and the properties of her
receiver.
6.2.1. Runaway Sexual Selection
Fisher’s hypothesis of runaway sexual selection is best illustrated with an
example. Suppose that there is heritable variation for a male signal, such as
simple song or complex song, and a female preference, such as a preference
for complex song or a lack of such a preference. The genes controlling trait
252
M.J. Ryan and N.M. Kime
generated considerable interest in sexual selection theory (e.g., Andersson
1994), but its generalities can be applied to the more general problem of
signal–receiver evolution. As part of this theory of runaway sexual selection, Fisher (1930) was the first to suggest that linkage disequilibrium plays
an important role in the evolution of sexually selected male signal traits and
female preferences for traits. It has been more recently applied to “good
genes” theories of sexual selection (Pomiankowski 1988; Grafen 1990).
The crux of linkage disequilibrium is that traits that are not under selection can evolve if they are genetically correlated with traits that are under
selection. In the parlance of evolutionary genetics, the trait subject to selection is said to be under direct selection. If another trait variant is more likely
to be associated statistically with the trait under direct selection than
expected by random chance, the two traits are genetically correlated or in
linkage disequilibrium. The second trait, not subject to direct selection, is
under indirect selection by virtue of its genetic correlation with the first trait
and can evolve as a correlated response to the trait under direct selection.
It is the observation that a trait can evolve even if it is not subject to
direct selection that makes linkage disequilibrium especially relevant to
sexual-selection studies. In lekking species of animals, males gather to signal
for mates and provide them with few resources besides sperm. It is assumed
that in many cases, regardless of with whom the females choose to mate,
the number of offspring they birth will be the same. Yet—and this is the
paradox—females sometimes assiduously choose their mates (in many
cases, the signal parameters that influence mate choice are well-known
(Ryan and Keddy-Hector 1992; Andersson 1994)), with only a few males
on the lek gathering a majority of the mating success. How could such
preferences evolve if there is no difference in the reproductive success of
females exerting different preferences or, for that matter, no difference in
the reproductive success of females exhibiting a preference or mating at
random? Two major hypotheses, runaway sexual selection and good genes
selection, have been offered as solutions to this paradox, and both are
dependent on indirect selection and linkage disequilibrium. Even though
this problem of evolution through linkage disequilibrium is usually
addressed within the context of male trait/female preference evolution, in
many cases the male trait is a long-distance signal and the female preference emerges from the interaction of this stimulus and the properties of her
receiver.
6.2.1. Runaway Sexual Selection
Fisher’s hypothesis of runaway sexual selection is best illustrated with an
example. Suppose that there is heritable variation for a male signal, such as
simple song or complex song, and a female preference, such as a preference
for complex song or a lack of such a preference. The genes controlling trait
252
M.J. Ryan and N.M. Kime