Sexual Selection

• Sexual Selection is a form of natural selection acting on differences in the number or identity of mates.
• Anisogamy: a system of sexual reproduction that involves the union of two gametes that differ in size.

  • likely arose via disruptive selection against intermediate-sized gametes
• Males produce many small, motile gametes (sperm).

  Females produce fewer, larger, nutrient-rich, nonmotile gametes (eggs).
• Isogamy: a system of sexual reproduction that involves similar size gametes and the ancestral condition.
• Bateman's Principles:

  • male reproductive success increases with the number of mates, females does not.
  • males show greater variance in reproductive success than females.
  • the sex with the greater variance will be more strongly affected by sexual selection.
• Parental Investment Theory: the sex that has the greater investment in offspring production should be choosier when it comes to mates.
• You are studying predation mortality in a population of small birds and notice that individuals in groups of less than five individuals suffer higher mortality than those in groups larger than five individuals. This observation is consistent with which of the following ideas? 

  Allee Effect: individual fitness may increase when the population density increases
• What distinguishes a territory from a home range?

  The presence of absence of active defense.
• Intrasexual Selection: competition between males for access to females, which can lead to the evolution of traits that increase a male's chances of mating
• Intersexual Selection: Mate choice based on physical traits of the partner.
• Weapons and Intrasexual:

  • Only male dung beetles have horn-like projections. Males fight to defend dung piles where females mate and raise offspring.
  • The maintenance of long horns can be explained by sexual selection driven by male-male competition. 
• Long-tailed Widowbird Intersexual:

  • Long-tailed widowbirds have long tails just because females are attracted to them. 
  • Their tails are not used in any type of competition or intimidation of males, but just for the attraction of females.
• Ornaments and Peacock Intersexual Selection:

  • Male peacocks aggregate on leks, where they display to females.
  • Larger males with longer tails were most successful in defending a display site. Males performing the most displays and with the most ocelli, obtained the most copulations.
  • Male-male competition and mate choice are both important in the evolution of peacock tails.
• Models Explaining the Existence of Mate Choice:

  1. Sensory Bias
  2. Direct Benefit
  3. Good Genes
  4. Fisherian Runaway
• Sensory Bias: mating preferences are a byproduct of pre-existing biases in a female’s sensory system.
• Direct Benefits: mating preferences exist because females can directly benefit by being choosy. 
• Good Genes: exaggerated sexual traits are costly to produce, and therefore must be a reliable indicator of male genetic quality. Females benefit indirectly in the form of “good genes” passed to their offspring.
• Fisherian Runaway: an evolutionary process, operating by a positive feedback loop, in which a female preference and the associated male trait are both present in offspring- can lead to wild exaggeration despite viability costs.
• Sensory Bias Example:

  Orange Guppies

  • Female mating preference could be linked to a preexisting preference for orange food objects. 
  • The strength of preference for orange correlated with a female mating preference for orange.
• Direct Benefits Example:

  Nuptial gifts in Hanging Flies

  •  Nuptial Gift: physical resource such as a food item that a male provides to a female. 
  • The larger the resource you give, the more time you get mating with the female, up to a certain point. 
• Hamilton-Zuk Hypothesis:

  • Proposes that sexual ornaments are indicators of parasite and pathogen resistance. 
  • What is “good” is always changing. Oscillations through time about the good and bad genes. 
• Fisherian Runaway:

  • An evolutionary process, operating by a positive feedback loop, in which a female preference and the associated male trait are both present in offspring- can lead to wild exaggeration, in principle, despite viability costs. 
• Define anisogamy and explain why it is expected to lead to sexual selection.

  Anisogamy is when the eggs and sperm are two different sizes. This will lead to sexual selection bc one parent will be more invested in offspring production and values every reproductive choice. Females can be choosy about mate preference and will never want for sperm.
• In what fundamental way do game theory models differ from opportunity models? 


  Game theory models are concerned with other individual's choices and how they can affect your own. In Optimality models, there are no concerns about what other individuals are doing because their choices wont affect you.
• Mate Guarding:

  • post-copulatory sexual selection
  • blue warblers protect their female after copulation.
  • females are promiscuous and can mate with multiple males. This means that males risk raising another male's offspring.
  • Nests with males who spent more time mate-guarding had fewer extra-pair offspring.
• Sperm Competition:

  • black-winged damselfly
  • male penises have barbs on them.
  • when males mate with a female, the barbs pull out all of the other sperm in order to ensure their sperm have a greater chance of fitness.
  • paternity assurance can be enhanced through frequent copulations
• Cryptic Female Choice: occurs when multiple-mated females influence the relative fertilization success of the sperm from each male.

  • females Orb spiders commonly mate with two males and then choose the sperm that they believe will be the best choice for them/ and their offspring.
• Alternative Mating Strategies:

  • when males cannot effectively compete, selection will favor the evolution of alternative mating strategies.
  • Sneaker Males will steal females from competing males.
• The Hamilton-Zuk hypothesis predicts which of the following?

  High parasite loads reduce the expression of secondary sexual traits.
• Sexual Conflict: occurs whenever there are differences in the evolutionary interests of males and females that revolve around reproduction.
• Ways in Which Females Influence Male Reproduction:

  • egg investment
  • mate choice
  • egg fertilization
  • offspring investment
• Ways in Which Males Influence Female Reproduction:

  • resources transferred to females
  • elaborate courtship
  • sexual coercion
• When the evolutionary interests of males and females are aligned (less sexual conflict), organisms are likely to be less promiscuous. If sexual conflict is completely absent, this can lead to monogamy.
• How can Male and Female Reproductive Interests Align?

  • when successful offspring production requires both parents.
  • if individuals are unlikely to ever encounter another mate
  • if territory defense requires both males and females
  • if males must guards females to sire any offspring
• Monogamy in California Mice:

  • Families with fathers removed had lower offspring success.
  • Male care enhances reproductive success in monogamous mice.