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The relationship
between sexual
selection and
human reproductive
behaviour
Male
Female
Male
Female
Male
Female
Sexual
Dimorphism
The differences of size,
colour, voice and so on
between the two sexes in
different animal species is
known scientifically as
sexual dimorphism.
Learning Objectives
1) To be able to understand how and why
males and females differ, and the results
this has on sexual selection.
Success Criteria
1) Complete pages 17-19 of your booklet.
2) Analyse lonely hearts columns as an
indication of our evolutionary past.
Challenge Create a plan for the essay
for sexual selection on page 20 of your
booklet.
Natural Selection
• Sexual dimorphism is maintained by the
counteracting pressures of natural selection and
sexual selection.
• Presumably, increased sexual dimorphism means
males are brighter and more conspicuous,
leading to increased predation.
• So long as the reproductive benefits of the trait
due to sexual selection are greater than the costs
imposed by natural selection, then the trait will
increase in the population.
• The benefits are a larger number of offspring,
while natural selection has the cost in the form of
reduced survival.
• This means that even if the trait causes males to
die earlier, the trait is still beneficial so long as
males with the trait produce more offspring than
males lacking the trait.
Sexual Selection
• Males tend to be the brighter of the
species.
• For example, the peacock. The tail of
the male you would not expect to be
naturally selected, so it must enhance
reproductive success in some way.
• Key terms:
– Intrasexual selection (mate competition)
– Intersexual selection (mate choice)
Sexual Selection
• Intrasexual selection
– Members of one sex compete with each other
for a member of the opposite sex. The victor
will be able to mate and pass on their genes.
• Intersexual selection
– There is a preference for a trait in a member
of the opposite sex, for example, a brightly
coloured tail. By mating with individuals with
the chosen trait, the trait will be passed on to
future generations.
• Draw an image on page 17 to illustrate
these concepts.
Lonely Hearts
• Analyse the lonely hearts columns.
• What do males want?
– In the short term...
– In the long term...
• What do females want?
– In the short term...
– In the long term...
Make some
notes and
attach into
page 17 of
your booklet
Sexual Selection
-evaluation-
• Being choosy requires time and energy and
this could impair survival.
• Random mating is stupid mating, because if
you are not choosy you will end up with lowquality offspring:
– Unattractive
– Unhealthy
• These low-quality offspring are then more
likely to produce low-quality offspring in
turn.
Short-term mating
preferences
• Men (according to parental investment theory)
desire casual sex, and sex early in a
relationship. Over the course of a year males
can impregnate a number of women.
• Women are not so affected by these
evolutionary desires. Woman can only fall
pregnant once every 9 months.
• According to Buss and Schmidt (1993) men
lower their standards in the short-term!
AND...show a decrease in attraction following
sex in order to make a speedy departure.
Short-term mating preferences
- evaluation • Clarke and Hatfield (1989)
• Men and women experimenters approached students on
a college campus. Said “I find you very attractive would
you...
– go on a date with me?
– go back to my apartment with me?
– have sex with me?
• Women
– 0% would have sex
– 6% would go back to the apartment
• Men
– 75% would have sex
– 69% would go back to the apartment
• Men have a desire for short-term mating, have a desire
for sexual variety, have little time between meeting and
intercourse, and are willing to have sex with strangers.
Long-term mating
preferences
• Investment in offspring is high.
• It pays to be choosy for both sexes.
• Buss (2003) found women to be attracted
towards males...
–
–
–
–
who have resources
can physically protect her
show promise as a good parent
are compatible enough to ensure minimal costs
to her or the offspring
• Males will look for signs of fertility so as not
to waste resources.
Long-term mating preferences
- evaluation • Buss (1989)
– 10000 participants
Excellent sample size
Not culturally bias
Supports AO1
Individualist and
– 37 cultures
research
collectivist cultures
– Interested in what males and females look
for in a marriage partner. Found:
• Women want males with good prospects
(resources)
• Men placed higher importance on physical
attractiveness (and physical fertility health)
• Males chose younger women (i.e. fertility)
• Both sexes want kind, intelligent and
dependable mates
IDA
• Research into mate choice lacks validity
because it tells us about what we prefer,
but not what actually happens in real life.
• But many research studies do support
the suggestions, e.g. Buss (1989) studied
marriage in 29 cultures and found that
men do choose younger women, and
after divorce men tend to remarry women
significantly younger than them.
IDA
• Research into short-term mating is
gender bias as men could not have
evolved to have a desire for short-term
mating without willing females.
• There must be some benefits (e.g.
leaving a poor relationship, or to
produce genetically diverse offspring)
Essay Planning
Learning Objectives
1) To be able to understand how and why
males and females differ, and the results
this has on sexual selection.
Success Criteria
1) Complete pages 17-19 of your booklet.
2) Analyse lonely hearts columns as an
indication of our evolutionary past.
Challenge Create a plan for the essay
for sexual selection on page 20 of your
booklet.