Lecture 15

• Sexual selection


  One criticism of natural selection that Darwin anticipated was that it could not easily explain traits that hinder survival (e.g., peacock's tail)
• Sexual selection


  In The Descent of Man (1871), Darwin elaborated his theory of sexual selection
• Sexual selection


  Variance in reproductive success results in selection on traits that increase one's ability to win mates
• Sexual dimorphism


  Caused by the two sexes having different optimal reproductive strategies
• Two main processes of sexual selection

  • Male–male competition for females
  • Female choice of males
• Charles Darwin, 1860: 'The sight of a feather in a peacock's tail, whenever I gaze at it, makes me sick!'

  
• Females


  Tend to invest heavily in fewer, larger gametes (eggs)
• Males


  Tend to invest less in producing many smaller gametes (sperm)
• Difference in investment in offspring

  Creates conflict in reproductive strategies
• Female's fitness


  Limited by her own egg/offspring production
• Mating with multiple males


  Generally provides less fitness benefit for females
• Females


  Waste valuable eggs if they mate with a low-quality male, so females are expected to be choosy
• Male's fitness


  Limited by how many eggs he can fertilize
• Mating with more females


  Can easily increase a male's fitness
• Sperm


  Cheap to produce, so the cost of mating with a low-quality female is low
• On average, making eggs may be ~1000 times more metabolically expensive than making sperm
• Variance in reproductive success

  Males have much higher variance than females in many species
• Females


  Generally have about as many offspring as they are physiologically capable of producing
• Variation


  The fuel for selection, so the sex with the greater variance in mating success will likely experience stronger sexual selection
• If the variation is heritable, selection will result in evolution
• Strong selection on males

  For mating success, may result in evolution of traits for male–male competition or female attraction
• Operational sex ratio


  When female biased, sexual selection may act on females
• Seahorses and pipefish


  Females lay eggs into a male's brood pouch, and he provides care until the eggs hatch
• Females


  Have higher variance in reproductive success and would experience stronger sexual selection
• Sexual dimorphism


  A difference between the sexes; it ranges from minor to extreme
• Intrasexual selection


  Competition between members of the same sex (usu. males) for access to members of the opposite sex
• Intrasexual selection


  Includes physical combat and sperm competition
• Physical combat


  • Male–male competition among ruminants has led to the evolution of headgear that is used as sexual weaponry
• Physical combat


  • Antlers, horns, pronghorns, & ossicones
• Physical combat


  • Sexually dimorphic horns or tusks are also found in other mammals, some insects, fish, reptiles, dinosaurs, etc.
• Sperm competition


  Competition between males may continue even after copulation
• Sperm competition


  If a female mates with multiple males, their sperm may compete inside the female by racing to fertilize the egg(s)
• Sperm competition


  May result in evolution of larger male ejaculations and/or changes to sperm properties (e.g. faster swimming)
• Sperm competition


  • Males may develop penile structures for removing the sperm of other males (e.g., in damselflies)
  • Males may also deposit a copulatory plug that plugs or glues a female's genital tract closed
• Testes size among primates

  Most variation is explained by body size, but a significant proportion is explained by mating system

  
• Polygamous species


  Have larger testes for their body size than monogamous species, consistent with the hypothesis that larger testes are an adaptation for sperm competition
• Alternative mating strategies: the "sneaker male"


  When there is intense competition among males over access to mates, alternative "sneaky" mating strategies may evolve
• Alternative mating strategies: the "sneaker male"

  • Smaller, less dominant males may try to obtain access to females while the dominant male is not looking
  • In some species (e.g., bluegill sunfish, wrasse, cuttlefish), sneakers mimic females to fool the larger male
• Infanticide


  In some situations, selection may favour killing competitors' progeny
• Infanticide


  • Male lions frequently do this when they immigrate into a pride and take it over