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DAVID W. HALL ET AL.
A second source of selection on preferences appears when in Expression (2) would no longer be multiplied by 1/2, since
females receive a direct benefit (or cost) from mating with it would now be expressed in both sexes.
certain males. Examples include when males provide parental
What is the ultimate fate of a population if a runaway is
care or when diseases are transmitted between mates. A third triggered? The results developed here tell us about the local
type of direct selection on preferences is caused by female stability of the equilibrium, but nothing about what will hap-
search costs (Kirkpatrick 1985; Pomiankowski 1987). Search pen to it if it does run away. Several outcomes are possible.
costs might favor females with no preference or alternatively One is that the population will arrive at a new equilibrium.
might favor females with extreme preferences if being sen- A second possibility is that the trait and preference will be-
sitive to certain signals makes it easier to locate males. Fe- come entrained in coevolutionary cycles (Iwasa and Pom-
males might, for example, find a male more easily in an iankowski 1995). It is impossible at this time to evaluate
environment rich in distracting stimuli if they are strongly either empirically or theoretically which outcome is likely to
tuned to conspicuous colors and sounds. These examples sug- be more common.
gest that some form of direct selection acts on most if not
all female mating preferences (Kirkpatrick 1985; Kirkpatrick
and Ryan 1991). Support for this view comes from selection
experiments in which mating preferences frequently evolve
as a side effect of selection directed at other traits (for review
see Rice & Hostert 1993).
Direct selection of preferences has two important effects
on the outcome of sexual selection. First, it determines where
the equilibrium for the female mating preference lies (Lande
ACKNOWLEDGEMENT
We would like to thank M. Whitlock and two anonymous
reviewers for valuable comments on the manuscript. This
work was supported by National Science Foundation grant
DEB-9973221 to MK.
L
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⌬
p¯ equation Rice, W. R., and E. E. Hostert. 1993. Laboratory experiments on
p