Two mechanisms for the maintenance of polymorphism
in genes that cause homosexuality have been
most frequently mentioned in evolutionary biology
literature: overdominance and frequency-dependent
selection via kin altruism. The former mechanism assumes
that genes inducing homosexuality provide superior
fitness in heterozygous conditions, for example, men
heterozygous for a homosexual gene may have higher
success in attracting women and/or their sperm may have
a competitive advantage over that of other men (e.g.
Hutchinson 1959; Weinrich 1987; Kirsch & Weinrich
1991; MacIntyre & Estep 1993; Miller 2000). The kinaltruism
mechanism assumes that homosexuals assist their
close relatives, thereby increasing their own inclusive
fitness (Trivers 1974; Pillard & Bailey 1998). A third
mechanism, which was briefly mentioned by Hammer &
Copeland (1994; see also McKnight 1997; Pillard &
Bailey 1998) but that has never been rigorously explored
previously, is a sexually antagonistic selection (e.g. Rice
1984; Rice & Holland 1997; Arnqvist & Rowe 2005)
under which alleles that decrease fitness of one sex are
maintained in the population because they increase the
fitness of the other sex. The potential importance of this
mechanism is highlighted by recent data which indicate
that female maternal relatives of homosexuals (Camperio-
Ciani et al. 2004) or relatives of gay men for both maternal
and paternal lines (King et al. 2005) have increased
fecundity.
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