Sexual Selection and Human Reproductive Behaviour

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Sexual Selection

Sexual selection -  characteristics that increase reproductive success. These are then passed on and may become exaggerated over succeding generations of offspring, in other word...where members of one biological sex choose mates of the other sex to mate with (intersexual selection), and compete with members of the same sex for access to members of the opposite sex (intrasexual selection).

Human reproductive behaviour: where individuals have the opportunity to reproduce and increase the survival chances of our genes. In other words...the physical features and characteristics of their mate choice can be used to pass on to the next offspring.

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Anisogamy

Anisogamy - refers to the differences between males and female sex cells called gametes.

Males - sperms

  • small
  • highly mobile
  • requires less energy to produce
  • produced continously in vast quantities from puberty to old age

Females - Ova's (eggs)

  • large
  • produced at intervals for a limited number of fertile years
  • requires huge amount of energy to produce

Anisogamy leads to two different mating strategies such as inter-sexual and intra-sexual selection due to the differences in males and females.

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Inter-sexual selection (INTER-**)

  • Inter-** is selection which occurs between the sexes
  • this mating strategy is preferred by females as its quality over quantity
  • Ova's require more energy to produce than sperms as well as ova's being more rare than sperms therefore females want a partner who is genetically fit who is able to provide her with the resources for her offspring. This means that women are more choosier when it comes to choosing a partner.
  • Runaway process (Ronald Fisher) is where females mate with a male who has a desirable characteristic and this trait is inherited by her son. This increases the likelihood that successive generations of females will mate with her offspring.
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Intra-sexual selection (INTRA-**)

  • Intra-** is selection within the sexes.
  • Preferred strategy by males (quantity over quality)
  • Intra-** refers to the competition between males to be able to mate with a female. The winner of the competition reproduces and gets to pass on to his offspring the characteristics that contributed to his victory.
  • Intra-** has behavioural and psychological consequences, for example for males to acquire fertile females and protect them from competing males, that may benefit from behaving aggressively.
  • Intra-** suggests that males like to mate with as many fertile females as possible. This is because of the minimal energy required to produce enough sperm to actually fertilise every woman on earth and the lack of responsibilty men have when looking after the baby because stereotypically its the mother who is 'suppose' to look after it.
  • A behavioural consequence of this strategy is having a distinct preference for youth such as having certain facial features as well as having a certain body shape.
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Evaluation of Sexual Selection

  • There is research done which supports anisogamy. David Buss carried out a survey over 10,000 adults in 33 countries. The questions were based on age and variety of attributes that evolutionary theory predicts should be important in partner preference. He found that female respondents placed greater value on resource-related characteristics such as good financial prospects and ambition. Males on the other hand valued reproductive capacity such s good looks and chastity and preferred younger mates, more than females did.
    • These findings show how there are differences in mating strategies used by the sexes because of anisogamy. They also support the predictions made about partner preference made by sexual selection theory.
    • These findings also have good cultural validity as it was done in 33 countries.
  • There is research which supports inter-sexual selection which was done by Russell Clark and Elaine Hatfield. They found that female choosiness is a reality of heterosexual relationships. Their experiment consisted of psychology students at university. These student were sent out acros the campus and asked other students this question "I have been noticing you around the campus. I find you very attractive. Would you go to bed with me tonight?" - they found that no female students agreed to the request, whereas 75% of males did.
    • These findings supports evolutionary theory because it suggests that females are choosier than males.
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Evaluation of Sexual Selection

  • Sexual selection ignores social and cultural influences. Social change has consequences for women's mate preference (Bereczkei) because women now choose their job careers over having a family. Also, because women now work it means they no longer depend on the man to provide for them.
    • These findings show that mate preferences are the outcome of a combination of evolutionary and cultural influences.
  • There is support for waist-hip ratio research. Males will show a preference for a female body shape that signals fertility, Devendra Singh studied this - she found that males prefer the ratio of waist to hip sizes - males prefer wider hips and narrow waists as this is attractive to them because it conveys an 'honest signal' (its hard to fake) that the woman is fertile but not pregnant. 
  • Lonely hearts research - David Waynforth and Robin Dunbar studied lonely hearts advertisements in American newspapers. They found that women more than men tended to offer physical attractiveness and indicators of youth. Men, on the other hand, offered resources more than women did and seeked for youth and physical attractiveness. 
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