Sexual Selection––#

SEXUAL SELECTION

  1. Introduction

    1. When Darwin published Origin of Species in 1859, he was at a loss to account for certain secondary sex characteristics
    2. How could natural selection favor the dramatic differences between sexes that are often observed?

      1. Male and female peafowl
      2. Male and female elephant seals
      3. Male and female rhinoceros beetles

    3. In 1871 he dealt with this problem in his book, The Descent of Man, by proposing a process he called sexual selection
    4. The theory of sexual selection seeks to account for the origin of secondary sex characteristics as the result of selective pressure exerted by individuals of the same species on one another
    5. Initially his ideas were rejected by the scientific community, but today they are considered important

  2. Parental investment

    1. Before we can understand sexual selection, we need to understand parental investment
    2. Parental investment is whatever increases the chances of offspring survival at the cost of parents’ ability to produce other offspring (Robert Trivers)

      1. Energy drain due to gamete production
      2. Energy put into parental care
      3. Risk of predation while caring for young

    3. Often the female investment is disproportionately large compared to that of the male

      1. Gamete production in birds
      2. Parental care in mammals

    4. By contrast, male parental investment is often relatively small

      1. Gamete production
      2. Parental care in mammals

    5. Thus, benefit/cost ratio for parental care is greater for females than males
    6. Thus, strategies for increasing “fitness” should be different for the two sexes

      1. Males should copulate with as many females as possible
      2. Females should be choosy about mate
  3. Sexual selection

    1. Male-male competition (intrasexual selection)

      1. “When the two sexes follow exactly the same habits of life, and the male has more highly-developed sense or locomotive organs than the female, it may be that these in their perfected state are indispensable to the male for finding the female; but in the vast majority of cases, they serve only to give one male an advantage over another, for the less well-endowed males, if time were allowed them, would succeed in pairing with the females; and they would in all other respects, judging from the structure of the female, be equally well adapted for their ordinary habits of life. In such cases sexual selection must have come into action, for the males have acquired their present structure, not from being better fitted to survive in the struggle for existence, but from having gained an advantage over other males, and from having transmitted this advantage to their male offspring alone” (Descent of Man, p. 248-249)

      2. Logic

        1. Males should mate with as many females as possible
        2. The sex ratio is often 1:1
        3. Therefore, females are a valuable resource for which males must compete
        4. This male-male competition serves as a selective force on males

      3. Male secondary sex characteristics and behaviors are often suited for this male-male competition

        1. Horns, antlers, strong muscles
        2. Ground nesting bees (Centris pallida)––males dig for emerging females
        3. Harems and sequential mating
        4. Hierarchies
        5. Territoriality

      4. Indeed, these traits and behaviors are thought to be enhanced through male-male competition.

    2. Female choice (intersexual or epigamic selection)

      1. Females should be picky about their mates––and, indeed, they seem to be picky
      2. Female pickiness serves as a powerful selective force on males
      3. Examples

        1. Tests of male strength

          1. Burrowing bees, Centris pallida
          2. European toad––bigger males dislodge smaller copulating males
          3. Copulatory tie in canids
          4. Female mallards signal males to attack other males
          5. Elephant seals––the female copulatory call incites male attack

        2. Male attractiveness

          1. Widowbird tail length
          2. Peafowl

    3. The enhancement of characteristics by sexual selection may become detrimental to the individual in terms of natural selection

      1. Irish deer antlers
      2. Peacock tail

  4. Mating systems

    1. Interactions between sexual selection and the environment determines the type of mating system
    2. Types of mating systems

      1. Monogamy

        1. Male able and willing to make significant parental investment
        2. Common among birds (>90% of bird species)
        3. Uncommon in mammals

      2. Polygamy––Evidence of sexual selection most apparent

        1. Polygyny

          1. Very common among mammals
          2. Types

            1. Parental investment polygyny––male provides significant resources

              1. Asian honeyguides
              2. Redwings

            2. Pure dominance polygyny––male provide few or no resources

              1. Elephants
              2. White-bearded manikans

                1. Lek species
                2. In one lek of 10 males, one male got 75% of the matings
                3. Only due to right perch

        2. Polyandry

          1. Rare as predicted by Trivers’ arguments
          2. Occurs when it is advantageous for male to be the only parent investing in offspring

            1. High predation rate
            2. Numbers of females fall
            3. Male guarding behavior is superior to female’s

          3. Types

            1. Resource defense polyandry––jacana
            2. Pure dominance polyandry––northern phalarope

    3. Promiscuity

      1. Parental care by male is unnecessary
      2. Occurs in some rodents, etc.

  5. Summary

    1. Sexual selection serves as an environmental force on the opposite sex
    2. It is a powerful form of natural selection