have two main benefits: either females gain for their sons those characteristics
that make a male attractive or successful in intrasexual competition, or
females gain viability genes for all their offspring (Andersson 1994). “Good
gene” arguments are often linked to the evolution of display characters since
female choice is often based on the size or elaborateness of male sexual ornaments
or the intensity of males’ displays. These are believed to indicate
viability or fitness, but how? The most influential idea is that selection favors
the evolution of signals that are costly to the signaler because these honestly
reflect the signaler’s quality (Zahavi 1975, Grafen 1990). The issue of signal
costs will be returned to later in the chapter since the evolution of such
“handicaps” may have direct effects on population viability and extinction
probabilities. However, the fitness benefits for individuals from mate choice
(reviewed by Møller, Christe, and Lux 1999; Jennions and Petrie 2000) are
probably concerned principally with the coevolution of parasites and hosts
(Hamilton and Zuk 1982). There is currently much interest in choice as it
relates to genetic variation in the major histocompatibility complex (MHC),
a hypervariable region of the genome concerned with immune function
(reviewed by Jordan and Bruford 1998).
Extensive research on mice shows that mates are chosen on the basis of
their genetic difference from the subject and that this information is
obtained using odors mediated by MHC variation (Potts et al. 1991). The
fact that the MHC is a region concerned intimately with immune function
suggests that the evolution of dissassortative female choice may be favored by
promoting increased disease resistance; for example, through heterozygote
advantage. Alternatively MHC variation may simply act as a polymorphic
marker to minimize inbreeding (Pusey and Wolf 1996).
A problem for arguments that invoke genetic benefits for female choice is
that strong directional selection due to female choice should have depleted
any genetic differences among males (as in the case of a reduction in variation
due to mating skew, as already outlined).Why is there any genetic variation left
among males in the population? This problem, the so-called lek paradox,
remains one of the outstanding problems in evolutionary biology and could
also have direct consequences for population viability. Theoretically, there
should be no additive genetic variance in fitness-related traits (Fisher’s
fundamental theorem), and where selection is strong, as it is in sexually selected
traits, then additive genetic variance should be lower than in nonsexually
selected traits. It is therefore surprising that sexually selected characters
show higher levels of additive genetic fitness than characters not under sexual
selection (Pomiankowski and Møller 1995). What mechanism involving
female choice could promote as well as remove genetic variance in fitnessrelated
traits? The answer has practical as well as theoretical significance if
the variation produced affects population viability. A possible mechanism
(M. Petrie, 2002, pers. comm.) is that female choice could support a higher
than normal mutation rate if the mutational load can be revealed in a display
character. Simulation modeling by G. Roberts and M. Petrie (2002, pers.
comm.) suggests that if females can select males who possess beneficial mutations
but who carry fewer deleterious mutations, then mutation rates 10
times those under random mating can be sustained. The idea that female
choice can maintain mutation rates provides a self-sustaining solution to the
lek paradox and predicts a greater level of evolvability in sexual populations.
This has practical consequences for population conservation since the
persistence of lek breeding may thus be important for population viability.
Lekking in topi is becoming increasingly rare and persists only in the few
remaining high-density populations. In the Mara ecosystem in Kenya it exists
only where grassland is lightly utilized (as inside the Masai Mara Game
Reserve) but not where it is intensively grazed (as outside the reserve with
large densities of livestock). The reason may be that leks form where topi
cluster in short-grass patches for antipredator advantage (Gosling 1986); this
response occurs only where female topi are forced to avoid surrounding
long-grass areas during the resting period of the day and not where the
sward is uniformly short. If lekking is influenced by such relatively simple
habitat features, it may be possible to intervene to help retain this mating
system. Of course it would be desirable to do this in any case because such
striking behavior as lekking in topi deserves to be conserved in its own right.
But it is also possible that such intervention might conserve behavior that
selects for high levels of genetic variation, removes deleterious mutations,
and thus promotes population viability.
The possibility that patterns of mate choice may confer such important
genetic advantages also has general implications for conservation breeding
programs. At present most breeding programs of rare animals in captivity
simply attempt to maximize outbreeding to retain as much genetic variation
as possible. However, for the reasons already discussed here and by Wedekind
(2002a) the benefits of allowing natural choice should be given careful consideration.
In practice this could be achieved either by allowing females to
choose mates or by artificial selection of mates according to the sort of criteria
suggested by recent research on mate choice (e.g., using estimates of MHC
similarity). Recent research suggests that not only might natural mate choice
prevent inbreeding, it might also be driven by genetic compatibility between
potential mates that provides resistance against particular pathogens
(Wedekind et al. 1996, Rülicke et al. 1998). Although this latter possibility
requires further investigation before its consequences are implemented in
conservation breeding programs, there are already grounds for believing that
benefits may be derived from allowing natural mate choice. Thus, where possible,
and in species with appropriate mating systems, allowing choice should
supersede breeding principles based on maximizing outbreeding since
achieving high levels of heterozygosity may not outweigh the costs of accumulating
deleterious mutations. The possibility that all potential mates will
prefer one individual, thus leading to the prospect of severe inbreeding depression,
is unlikely because assortative patterns of mating should generally be
more common. Direct natural choice should be used wherever possible, but in
intensively managed systems this may not be possible. Examples include small
declining wild populations where intervention is essential, or captive populations
where the financial cost of providing a natural choice is high. In these
cases choice of olfactory signals (particularly scent marks, which can be frozen
and shipped among cooperating zoos) provides the greatest promise. These
odors provide subtle information about genetic variation in potential mates
(reviewed by Gosling and Roberts 2001) and they could provide powerful
measures of mate preference if used in properly designed choice assays.
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