Evolutionary explanations for partner preferences

• Who came up with sexual selection?

  Darwin (1874) came up with his theory of sexual selection, describing two processes through which it took place.
• Sexual Selection:

  • An evolutionary explanation of partner preference. Attributes or behaviours that increase reproductive success are passed on & may become exaggerated over successive generations of offspring.
  • Any genes which are advantageous for survival are naturally selected; any genes that promote successful reproduction are selected = sexual selection.
• Sexual Selection assumptions:

  • The most successful animals will end up with both physical & behavioural characteristics that will enable them to outperform rivals. We should be finely tuned for growth, reproduction & survival.
• Human Reproductive Behaviour:

  • Refers to any behaviours which relate to opportunities to reproduce & thereby increase the survival chances of our genes. It includes the evolutionary mechanisms underlying our partner preferences, such as mate choice & mate competition.
  • Spec refers to human reproductive behaviour - any inclusion of non-human reproductive studies can be used only to illustrate important points concerned with human behaviour.
• Anisogamy 


  Anisogamy refers to the differences between male and female sex cells (gametes) - it relates to mating strategies.
• Males (sperm)

  • Small
  • Highly mobile
  • Created continuously in large numbers (cheap) - therefore there is no shortage of fertile males, but a fertile female is a ‘rare resource’ 
  • Does not require a large amount of energy to produce.
• Females (eggs or ova)

  • Relatively large 
  • Produced at intervals for a limited amount of time during fertile years.
  • Small amounts made vs sperm because it requires a huge investment of energy (expensive).
  • Static
• Consequences of anisogamy?

  • Consequence of anisogamy is that there are plenty fertile males but fertile females are considered a rarer resource. This gives rise to two  different types of mating strategies:
  • A consequence of this for mate selection is that…
  • Anisogamy leads to 2 different mating strategies:

  Inter-sexual selection 

  Intra-sexual selection
• IntRA-sexual selection:

  • Selection of mates WITHIN sexes e.g. males compete with other males for mates - Mate competition, Quantity over quality.
  • One sex competes with members of the same sex for access to females. The ‘winner’ gets to pass on their genes to the next generation, so the characteristics/traits that contributed to their victory (e.g. height, cunning) are perpetuated.
  • Males strategy quantity over quality: Often its males competing for access to females as sperm is plentiful but fertile females are a limited resource and are more choosy.
• IntRAsexual Selection Part 2

  • Dimorphism = The obvious difference between males & females e.g. larger males have an advantage and are therefore more likely to mate.
  • Impact on partner preference: Intra sexual selection pressures lead to certain patterns of human reproductive behaviour:
  • Physical Consequences - males who are bigger win competition for mates, so size is selected in males. Signs of fertility are selected (e.g. narrow waist in humans indicate youthfulness) in females.
  • Behavioural consequences - male aggressiveness also helps win competitions so they are more likely to mate.
• IntER-sexual selection Part 1

  • Selection of mates between sexes (i.e. females select males/males select females) - Mate choice, Quality over quantity.
  • One sex is very choosy in selecting sexual partner based on particular preferences = preferences of one sex thus determines area in which other sex must compete - leads to development of traits (e.g. desiring height) that enable animals to attract members of the opposite sex (passed down over generations since ones with wanted traits mate and pass that trait down).
• IntER-sexual Selection Part 2


  • Trivers (1972): Female strategy quality over quantity: Thus females optimum mating strategy to select a genetically fit partner who able & willing to provide resources for them & their offspring coz females make greater investment of time, commitment etc. before, during & after birth, thus wants someone who can take care of them & give their offspring good traits.
• IntER-sexual Selection Part 3

  • Impact on partner preference:
  • Preferences of both sexes determine attributes that are passed on e.g. if height is considered an attractive male trait, it increases in the male population over generations because females choose the tallest males - the trait becomes exaggerated.
  • Over time this leads to taller and taller men being selected (runaway process) = Fisher (1930) sexy sons hypothesis
• Fisher (1930) - IntER-Sexual Selection

  Sexy Son Hypthesis - Females mate with males who have desirable characteristics. Their sons then inherit this 'sexy trait'. This increases the likelihood that the successive generation will mate with the females' offspring.
• Suggests that Females & males seek different things in relationships:

  Females seek:

  • Compatibility 
  • Ability to physically invest 
  • Show promise as a good parent 
  • Ability to invest financially in them & child

  Men seek:

  • Females who appear most fertile (broad hips etc)
• Females Seek?

  Females seek:

  • Compatibility 
  • Ability to physically invest 
  • Show promise as a good parent 
  • Ability to invest financially in them & child
• Males Seek?

  Men seek:

  • Females who appear most fertile (broad hips etc)
• Evolutionary explanations for partner preferences?

  • 
    
  • Sexual Selection
  • Human Reproductive Behaviour
  • Anisogamy
  • Intra & Inter - Sexual Selection
  • What Females & Males seek in relationships
• Sexual Selection?

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  A) ?

  
• Human Reproduction Behaviour?

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  A) ?

  
• Anisogamy

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  A) ?

  
• Consequences of Anisogamy?

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  A) ?

  
• IntRA-Sexual Selection?

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  A) ?

  
• IntER-Sexual Selection?

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  A) ?

  
• Fisher (1930)

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  A) ?

  
• Males & females seek different things in relationships?

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  A) ?

  
• Evolutionary explanations for partner preferences?

  • Sexual Selection
  • Human Reproductive Behaviour
  • Anisogamy
  • Intra & Inter - Sexual Selection
  • What Females & Males seek in relationships