Sexual Selection

• How does natural selection differ from sexual selection?

  Natural Selection (Darwin): survival of the fittest - advantageous characteristics are more likely to survive

  Sexual selection: the process of reproduction through mating strategies - high quality traits that help obtain mates are likely to have offspring
• Sociobiology

  A field of psychology which aims to explain social behaviour in evolutionary terms
• Adaptive behaviours

  Behaviours that lead to increased survival and reproduction of an organism (in sexual selection attraction is seen as adaptive)
• AO1: Anisogamy

  • The difference between male and female sex cells (gametes)
  • 'The sex which invests the most in producing offspring (females) becomes a limiting resource over which the other sex will compete (males)'
  • The unequal investment lies in the difference in the production of gametes
• How do the male and female gametes differ?

  Size, function

  • Sperm: Small in size, many (40-600 million), energetically cheap
  • Ovum: Large in size, relatively few (300), energetically expensive
• How do male and female differ in terms of offspring?

  Males: Technically can have an unlimited amount of offspring, very fertile

  Females: Have to nourish offspring for 9 months, as well as they physical strain, she can only have a limited amount of offspring
• AO1: Two types of sexual selection

  1. Intersexual selection - Between the sexes, the strategies that females use to select males
  2. Intrasexual selection - Within each sex, competitive strategies between males to be the one that is selected
• AO1: Intersexual selection

  The preference of one sex for the member of the opposite sex who has certain qualities (quality over quantity)

  • Females make a greater investment and commitment in the birth of their offspring, they are more particular in their choice of partner
  • They choose a partner who can offer resources (food, territory, protection, money status, ambition) because it enhances the female reproductive success
  • The 'attractive' traits gets passed onto their offspring
• How is a peacock a good example of intersexual selection?

  It is described as the 'runaway process', where adaptive features become exaggerated over many generations. Male peacocks have large blue and green features to attract females.
• 'sexy sons hypothesis'

  A female mates with a male who has desirable characteristics, and the 'sexy' trait is inherited by her son. This increases the likelihood that successive generations of females will mate with her offspring, guaranteeing future reproduction.
• AO1: Intrasexual selection

  The competition within males to mate with a female (quantity over quality)

  • If they win against other males, they are able to reproduce and pass on their characteristics - victory
  • They often act aggressively to their peers as a way to rival
• Dimorphism

  The obvious difference in appearance between males and females evolved over time as they bring an advantage in reproduction
• What are the sexual differences between males and females?

  • Different motivation
  • Women only need to find one man
  • The more females a male impregnates, they greater his reproductive success
  • Mating a poor quality males has poor implications on the offspring
  • Males lack post-coital responsibility (females carry baby)
• Psychological and behavioural consequences of sexual selection

  • Men benefits from behaving aggressively and thinking in a certain way
  • Men's preferences for youth and a sensitivity to indicators of youth as well as fertility
• Reproductive value

  The probability of reproducing in the future (long term)

  • 13 year old - high reproductive value
  • 30 year old - low reproductive value
• Fertility

  The probability of reproducing now (short term)

  • 13 year old - low fertility
  • 30 year old - high fertility
• AO1: Symons 1979

  Males prefers females who indicates youthfulness - physical characteristics (smooth skin, good muscle tone, full lips, healthy hair), behaviours (high energy levels, spirited character)
• AO3: Buss 1989

  • Explored what males and females looked for in a marriage partner using over 10,000 people from 37 different cultures
  • Females -> males: Looked for good financial prospects, ambition (resource-related characteristics because males can provide and take care of family/children)
  • Males -> females: Looked for physical attractiveness, youthful indicators (characteristics showing higher reproductive value because it implies females can carry more children)
• AO3: Evaluation of Buss's study

  • Cross-cultural study emphasised the unity in behaviours from anisogamy (further signifies partner preference from sexual selection theory)
• AO3: Clark & Hatfield (1989)

  • Investigate the difference in reproductive behaviour between men and women
  • Experimenters approached total strangers on a university campus and said "Hi, I've been noticing you around campus and I find you very attractive"
  • Three questions were asked, 1. Would you go on a date with me? 2. Would you get go back to my apartment 3. Would you have sex with me?
• AO3: Results of Clark & Hatfield's study

  1. Would you go on a date with me? (Males: 50%, Females: 50%)
  2. Would you go back to my apartment? (Males: 69%, Females: 6%)
  3. Would you have sex with me? (Males: 75%, Females: 0%)
• AO3: Evaluation of Clark & Hatfield's study

  • Supports the idea of sexual selection
  • Evidence that men have evolved psychological mechanisms to ensure success in short-term mating, including:

  1. a desire for sexual variety
  2. the tendency to let little time elapse before seeking sex
  3. a willingness to consent to sex with strangers
• AO3: Evaluation of sexual selection theory

  • Ignores social and cultural influences:
  • Social norms of sexual behaviours have rapidly changed over the past century
  • Availability of contraception
  • Women's role in the workplace: no longer dependent on men to provide (Bereczkei - women's mate preference may no longer be resource oriented)
  • Chang - Some partner preferences have changed over 25 years in China corresponding to social changes

  The evolutionary theory is stereotypical and must take in consideration cultural changes.
• AO3: Lonely hearts research - Wayneforth & Dunbar

  • Studied lonely hearts ads in American newspapers
  • Men and women would describe the qualities they desire in a partner, whilst cataloguing what they had to offer
  • The found women tended to offer physical attractiveness and indicators of youth, and men offered resources
  • Supports sexual selection theory
• AO3: Waist-hip ratio research

  • Males show a preference for the female body shape that signals fertility -> increased chances of reproduction
  • Singh (1993, 2002) studied this in terms of waist-hip ratio
  • Male preference is not the female body size in general, but the ratio of waist to hip sizes (ideal ratio = 0.7)
  • Wider hips and narrower waist = fertile but not currently pregnant ('child-bearing hips')