  Several studies question the adaptive value of female preferences for older
males. Theory and evidence show that older males carry more deleterious
mutations in their sperm than younger males carry. These mutations are not
visible to females choosing mates. Germ-line mutations could oppose preferences
for "good genes." Choosy females run the risk that offspring of older males
will be no more attractive or healthy than offspring of younger males.
Germ-line mutations could pose a particular problem when females can only judge
male trait size, rather than assessing age directly. I ask whether or not
females will prefer extreme traits, despite reduced offspring survival due to
age-dependent mutation. I use a quantitative genetic model to examine the
evolution of female preferences, an age-dependent male trait, and overall
health ("condition"). My dynamical equation includes mutation bias that depends
on the generation time of the population. I focus on the case where females
form preferences for older males because male trait size depends on male age.
My findings agree with good genes theory. Females at equilibrium always select
above-average males. The trait size preferred by females directly correlates
with the direct costs of the preference. Direct costs can accentuate the
equilibrium preference at a higher rate than mutational parameters. Females can
always offset direct costs by mating with older, more ornamented males.
Age-dependent mutation in condition maintains genetic variation in condition
and thereby maintains the selective value of female preferences. Rather than
eliminating female preferences, germ-line mutations provide an essential
ingredient in sexual selection.
