Biology Reference
In-Depth Information
females of the swordless platyfish and
Priapella
: when artificial swordtails were
appended to the tails of their males, females preferred them to their normal tail-less
males (Basolo, 1990, 1995). This suggests that these fish all have a pre-existing
female preference for tails and this was then exploited by some
Xiphiphorus
species.
Now, according to Fisher's runaway model, the preference and trait evolve together,
so female tail preference is predicted to be stronger in the swordtails. However,
female preference for swords is stronger in the genera that lack swords, suggesting
that female attraction to swords has
declined
as swords have evolved, just as predicted
by the chase-away model.
In
Schizocosa
wolf spiders, too, experiments show that tufts of bristles on the forelegs
of males have stronger effects on female sexual receptivity in species which lack the
tufts than in species with tufts (McClintock & Uetz, 1996). Once again, this suggests that
the evolution of a male trait has been accompanied by female resistance.
So, to conclude, we have three models for the evolution of female preferences by
sexual selection and there are examples which provide evidence for all three. Future
studies need to investigate the relative importance of the three different processes in
nature. It is possible that males of some species will have traits that have evolved by all
three processes; a male duck, for example, might have bright glossy plumage which
signals his good genes for parasite resistance, a long-tail which has been exaggerated by
the Fisher runaway process, and an elaborately shaped penis which has evolved to
enhance extra-pair matings, but which has provoked the 'chase-away' coevolution of
elaborate female genitalia as a counter-defence.
Different models
of sexual
selection may
apply to different
traits
Summary
Females usually invest more in gametes and parental care, so males spend more time in
the mating pool and successful males have a greater potential rate of reproduction. This
leads to males competing for females and females choosing among males based either
on the resources they provide or their genetic quality. Darwin's (1871) theory of sexual
selection was proposed to explain the evolution of traits that are of advantage in
competition for mates, either by force or by charm. There is good evidence that a male's
mating success is related to size and strength (e.g. elephant seals) or to ornaments
which attract females (e.g. long tails in widowbirds). Females may choose males that
provide the best resources, in the form of nest sites, food or paternal care. However,
females are also choosy even in cases where males provide only sperm. Here, elaborate
male displays may evolve by Fisher's runaway process (where the benefits of choice are
genetically attractive sons) or by honest advertisement of genetic quality (females gain
good genes, which increase viability of both sons and daughters). In cases where males
make a large contribution to parental investment, males may be choosy and females
may compete for mates.
Parker (1970c) showed that sexual selection continues after mating, as sperm from
rival males compete for fertilizations inside the female tract (sperm competition).
Females may gain both material benefits and genetic benefits from mating with more
than one male. This leads to sexual conflict, with male adaptations to improve success
in sperm competition and female adaptations involving sperm choice and resistance to
