Chapter 10.  Sexual Selection

• Sexual dimorphism is common in many animal species (and a few plant species), includes differences in
• size
• coloration
• mating behaviors
• Can natural selection explain sexual dimorphism?
• Some traits (e.g. bright feathers) would seem to reduce fitness.
• Darwin's solution: if there is heritable variation in traits affecting mating success, traits conducive to success will become more common. This is sexual selection.  (Note: sexual selection is not the same thing as mate selection, although the latter can result in the former.)
• Asymmetries in sexual reproduction are common, especially with regard to parental investment which is
• Typically high in females, includes expense of eggs, incubation, gestation, defense of young, etc.
• Typically low in males.  Sperms are cheap, there is rarely any paternal care of young.
• What limits reproductive success in females and males?
• Females: limited by number of eggs that can be produced or pregnancies that can be carried.  This leads to the prediction that females should be "choosy", resulting in intersexual selection.
• Males: limited by number of females he can mate with.  This leads to the prediction the males should be "competitive", resulting in intrasexual selection.
• Rough skinned newt (Figure 10.5) - results of investigation confirm predictions, males more limited by access to mate than females
• Broad-nosed pipefish (Figure 10.5) - an example of sex role reversal, in this case females are more limited by access to mates.
• Male-male competition resulting in intrasexual selection.
• Combat.  Example: marine iguanas (large males monopolize females).  See Figures 10.7-10.10, Table 10.1
• Sperm competition.  Many insect examples (to counter multiple copulations, tactics include producing more sperm, inserting plug, cleaning out competitors' sperm)
• Infanticide.  Example: Lions (males kill unrelated young)
• Female choice resulting in intrasexual selection.
• Do females really make choices based on male traits?  Example: female barn swallows prefer males with long feathers.  See Figures 10.16-17, Table 10.2
• How do females benefit from appropriate choices?
• Access to more resources
• Fathers with good genes produce more fit offspring.  Example: tree frogs, Figures 10.20-21, Table 10.3.
• "Sexy son" hypothesis.
• What abut humans?
• Yanomamo (Amazonia) - killers father more children. Figure 10.33.
• Polish men - tall men are more likely to marry than short men, tall fathers have more children than short fathers.  Figure 10.34.
• Sociobiology and evolutionary psychology - beware!