PSYCH 2TT3 Chapter Notes - Chapter 7: Parental Investment, Cichlid, Mate Choice

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23 Dec 2014
Chapter 7: Sexual Selection
Intersexual Selection and Intrasexual Selection:
Darwin’s work on sexual selection focuses on
intrasexual selection (members on one sex compete with
each other for access to the other sex) and intersexual
selection (individuals of one sex choose which
individuals of the other sex to take as mates)
Females are choosier b/c eggs are expensive to produce.
Female’s greater choosiness in mate selection should
translate into greater variance in the reproductive success
of males
Natural Selection will act against production of medium-
sized gametes if they are not very mobile compared to
small gametes and don’t provide a great deal of
resources to developing zygotes compared with large
gametes. After natural selection, only small gametes
(sperm) and large gametes (eggs) are produced
Secondary or epigametic sexual characteristics: males possess other traits that play an
important role in attracting mates (i.e. ornamental plumage, bright colours and courtship
displays) that are favoured by natural selection
Both intrasexual and intersexual selection play a role in monogamous, polygamous, or
polyandrous systems (generally, selection is weaker in monogamous)
Evolutionary Models of Mate Choice:
Evolutionary models of females choice can be
broken down into four classes: direct benefits, good
genes, runaway selection and sensory exploitation
Direct Benefits:
A model of sexual selection that favours females that
have a genetic predisposition to choose mates that
provide them with resources that increase their
fitness- the choosier the female, the better
Nuptial Gifts: prey presented by members
of one sex to members of the other sex
during courtship
Scorpionfly: females choose males with
largest gifts. Also decide how long they
will mate with male based on gift size.
Small gift = small copulation time. Large
gift = long copulation time. Females that
choose males with large gifts produce
more eggs and live longer because of
nutrition from gift and decreased time that
she would’ve spent hunting. For males,
searching for large gift is time consuming and dangerous.
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Good Genes and Mate Choice:
A model of sexual selection in which females choose males with good
genes. Females receive “indirect” benefits, in that their offspring receive
good genes
Pronghorn Antelopes: females choose
mates based of males ability to defend
harem. Females mate with a small subset
of males in their population leading to increase variance
in harem size and to males with large harem in prior
years siring a disproportionate number of offspring.
Males with large harems should have offspring that are
more likely to survive.
Honest Indicator: traits that are costly to produce are
more difficult to fake and hence truly indicate the
genetic quality of an individual
Parasite Resistance: females choose males with strong resistance to parasites (difficult to
fake trait). Look for bright
colouration (healthy) or dull
MHC = gene involved in disease
resistance. Few individuals have the
same MHC, therefore animals prefer
mating with other that have
dissimilar MHC, resulting in
offspring with strong immune
oMICE : females use odors to
determine MHC
oHumans : women found odors of shirts from males with
dissimilar MHC sexier, suggesting MHC has a significant
effect on females mate choice in humans
oStickleback Fish : prefer males with greater # of MHC alleles.
Experiment: expose females to two different water columns. 1.
Had male X swimming in it, 2. Had water from male X’s tank +
MHC peptides. When a male + females pair had a combined #
of MHC alleles below optimum, females preferred water that
was supplemented with MHC peptides. When a male + female
pair had combined # of MHC
alleles above optimum, female
preferred water that was not
supplemented with MHC
Runaway Sexual Selection:
group of
sharing the
same mate.
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