Evolutionary Explanations

• Sexual Selection

  A process of natural selection and the key factor underpinning human reproductive behaviour
• Trivers, 1972: 'Sexual Selection suggests evolution is driven by competition for mates and the development of traits that ensure a reproductive advantage'
• Sexual Selection

  • Individuals with traits that increase reproductive success (e.g. physical attractiveness, strength, height) gain an evolutionary advantage because they are more likely to attract a mate
  • Individuals with traits that enable them to outcompete rivals (e.g. physical strength) are also likely to gain access to a mate
• Reproduction and inheritance

  1. Individuals with advantageous traits attract a mate and successfully reproduce

  2. Their offspring inherit their parents' genes (e.g. for attractiveness or strength)

  3. These respective genes will be passed on to future generations

  4. Individuals without these traits are outcompeted, unable to mate, and therefore will not reproduce

  5. The genes underlying these traits will slowly disappear from the gene pool in future generations
• Intrasexual selection


  Same-sex contest, individuals outcompete members of the same sex to gain access to mates, usually a male strategy involving aggression, preference for many mates (polygamy), characteristics such as greater strength and cunning get passed onto future generations
• Intersexual selection


  One sex competes for attention of opposite sex, individuals evolve preferences for desirable qualities in the opposite sex, usually a female strategy which can involve choosiness, preference for a single mate, characteristics such as having access to resources and attractiveness will get passed on to future generations
• Anisogamy


  Differences between male and female gametes (sex cells), which result in different strategies for reproductive success
• Male gametes (sperm)


  • Numerous
  • Highly mobile
  • Replenished
• Female gametes (ova)


  • Large
  • Static
  • Limited
• There is no shortage of fertile males however fertile females are a 'rare resource'
• Reproductive strategy of males


  Seeking mates who are fertile and have high reproductive value indicated by age and hip to waist ratio
• Reproductive strategy of females


  Choosing a mate who is able to provide economic resources and support for her and her offspring
• Research supporting sexual selection

  • Evidence that reveals gender differences in human reproductive behaviour suggested by sexual selection is provided by Clark & Hatfield (1989)
• Clark & Hatfield (1989) study


  1. Male or female stooge approached an opposite sex stranger with an invitation to go to bed with them that night

  2. 75% of men agreed immediately to the request

  3. None of the women did
• Even in a society where a female can access her own resources, and contraception allows for sexual permissiveness


  Females remain choosy and males are considerably less selective
• Human reproductive behaviour is still driven by instinctive evolutionary pressures
• Anisogamy


  Differences in the size of male and female gametes
• Anisogamy


  Leads to different mating strategies in males and females
• Buss (1989) study


  • Surveyed over 10,000 adults in 33 countries
  • Asked about partner preference
• Females


  Valued resource related characteristics more than males (e.g. good financial prospects)
• Males


  Valued reproductive capacity (e.g. good looks and youth)
• These findings reflect sex differences in partner preference due to anisogamy
• The findings support predictions about partner preference derived from sexual selection theory
• The findings can be applied across different cultures reflecting fundamental human preferences which are not primarily dependent upon cultural influences
• Gender differences in partner preferences

  Not only due to sexual selection pressures (nature) but actually stem from socialisation and culture (nurture)
• Nurture factors in human reproductive behaviour

  • Women in cultures where their status and educational opportunities were low valued resources in a potential mate
  • Women with greater educational and financial freedoms were less likely to seek resources in a mate
• The example refutes the inter-sexual selection idea that women are attracted to men who can provide for their offspring
• Mixed mating strategies

  Females pursuing both a main partner and also copulating with a more masculine faced male when most likely to get pregnant
• Ovulating women


  Indicate an attraction to a more masculine face
• Non-ovulating women

  Prefer a feminised version of a male face
• Sexual selection

  • May favour females who pursue a mixed mating strategy
  • A female might choose a main partner whose feminised appearance suggests kindness and parental care
  • A female might also copulate with a more masculine faced male (indicating testosterone and strong immune system) when she is most likely to get pregnant
• The use of mixed mating strategies indicates inter-sexual selection is more complex than originally believed