ANIMAL BEHAVIOR
R. H. Wiley
28. Sexual selection
Why are females choosy and males competitive (in most species)? -- females (by definition)
produce nutritive gametes (eggs), which are metabolically expensive to replace -- their
reproductive success is usually limited by how many eggs they can produce (or young they
can nourish) -- male reproductive success is usually limited by how many mates he can obtain
-- production of gametes or nourishment of young does not usually limit male reproduction -in some species, a male's contribution to parental care (building nests, feeding young,
incubating eggs) does limit his reproductive success -- in these species males are choosy
about mates (seahorses that brood their mates' eggs in a pouch, sticklebacks that build and
tend a nest for eggs)
Natural selection results from differences in reproduction or survival of individuals
(phenotypes) with different genes in a particular environment
sexual selection is a special case of natural selection -- it results from differences in
reproduction as a result of differences in access to mates -- it differs from other forms of
natural selection in a crucial way -- it is self-accelerating -- once started it tends to snowball
Consider two loci (for simplicity) -- one affects female preferences for mates (P results in a
preference for males with some trait, like a long tail, and p results in no preference) -- one
affects the male trait (T results in a long tail, t does not) -- females with allele P tend to mate
with males with allele T, females with allele p mate at random, males with allele t have less
chance for mating than those with allele T -- in the next generation more individuals will
have allele T -- note what happens to allele P -- because females with P tend to mate with
males with T, their progeny (both male and female) tend to have both alleles (T alleles
might not be expressed in females, nor P alleles in males, but both male and female progeny
carry them) -- tendency of two separate alleles to occur in same individuals is called genetic
correlation
As generations pass, alleles T and P continue to occur together in the same individuals -consequently when females expressing allele P choose a mate with allele T, they usually are also
choosing a mate carrying allele P -- the more P alleles there are in the population, the greater the
mating advantage of males with T alleles, the faster allele T spreads, and the faster allele P
spreads (because of genetic correlation) -- so the P allele spreads itself -- in other words, spread
of P alleles is self-accelerating (produces positive feedback)
ANIMAL BEHAVIOR
R. H. Wiley
Sexual selection (continued)
Sexual selection can produce two paradoxical results
(1) Female preferences result in evolution of male traits that reduce their survival -higher mortality of males with preferred traits is balanced by higher success in mating -experiments have compared male barn swallows with shortened or elongated or
unmanipulated tail feathers -- birds with experimentally shortened tails do better than
unmanipulated males -- they are more likely to survive the winter and they bring larger
(more nutritious) insects to feed their young -- those with experimentally lengthened tails
do worse
(2) Self-accelerating feature of sexual selection allows purely arbitrary female preferences
to spread -- arbitrary preference is one for a male trait that has no advantages for male or
female other than access to mates
On the other hand, sexual selection is eventually stopped either (1) by sufficiently high
mortality of males with the preferred trait or (2) by any net costs to the female of
choosing
Female preferences cannot evolve (spread in a population) if females incur costs as a
result of expressing a preference -- for instance, preferences can take time and involve
risks of predation and consequently might reduce chances for reproduction or survival
Consequently, female preferences for advantageous traits (rather than arbitrary traits) are
more likely to spread -- two kinds of advantages for female preferences -- direct
advantages (because preferred males provide more parental care or better territories or
because choosing them involves less risk of predation or disease for the female) -indirect advantages or good genes (because preferred males have genes for greater
resistance to disease which offspring inherit)
Male barn swallows with longer tails are better parents (provide larger insects to feed
young), have greater genetic resistence to insect parasites of nestlings, and are more likely
to survive the following winter -- evidence for both direct and indirect advantages of mate
choice