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Zoology 445
Fall 2006
Sexual Selection: Handout
I. Introduction
One feature of many species that is not easily explained by natural selection is sexual
dimorphism. = females & males differ phenotypically.
1. Male cardinals are bright red, while females are drab.
2. Male peacocks have massive tails and are brightly colored compared to females.
Why do these traits evolve:
1. Finding a mate is critical for reproductive success in sexually reproducing species; results
in strong selection
2. Parental investment in offspring is often not equal for males and females.
Sexual dimorphism is a consequence of this unequal parental investment.
Note: In species where parental investment between the sexes is nearly equal, then sexes
tend to be similar phenotypically.
Differing optimal strategies when parental investment differs between the sexes.
1. Sex with the higher parental investment should be discriminating when choosing a mate.
(usually this is the female sex & term female choice)
2. Sex with the lower parental investment should mate as many times as possible. (usually
the male sex & termed male-male competition)
A. Sexual selection:
1. Intrasexual selection: When males can control access to females or control a resource
needed for mating (e.g., nest sites or food), then males compete with males for mates.
2. Intersexual selection: When males cannot control access to mates, then female choice
Note: No breeding system is exclusively one type of sexual selection. Each system has elements
of both intrasexual and intersexual selection.
II. Intrasexual selection
A. Combat
B. Sperm competition (see page 386 of text)
C. Infanticide (see page 387 of text)
A. Combat: Evolutionary result is sexual dimorphism where males are larger than females or
males have weapons.
Weapons: Bighorn sheep
Size: Marine iquanas (Amblyrhynchus cristratus) on the Galapagos Islands.
Text and fig. 10.8 explain that largest males have reduced survivorship because they have
difficulty obtaining enough food.
AM Jarosz
Zol 445
Large male size is driven by intrasexual selection
Fig 10.10a
Fig 10.10b
III. Intersexual selection (Female choice) When males cannot monopolize females or resources
needed for mating then they “advertise” for mates. Females choose mates based on male
Example: Barn swallow tails Studied by Anders Møller
Four treatments:
1.Cut tail feathers, shorten by 2cm and glue back on.
2.Control where tail feather is cut then glued back on.
3.Control male tail feathers are unaltered.
4.Cut tail feathers, lengthen by 2cm and glue back on.
Zol 445
Fig 10.16b Tail feathers of males average 15%
larger than females.
Two hypotheses for explaining trait on which females choose:
A. The display indicates that the male has “good genes”
B. Runaway selection due to historical natural selection or sensory bias.
A. Good gene hypothesis
1. Gray tree frog. Females prefer males with long call. Table 10.3 Book
Males with long calls seem to confer better growth in the offspring.
Zol 445
Broad-nosed pipefish – Males care for the young and invest more in the offspring. Therefore
they are choosy (male choice)
Zol 445
B. Runaway selection (Book also calls this the sexy son hypothesis)
See also page 398- 399 for sensory bias explanation.
Take home message:
Sexual selection explains sexual dimorphism and showy displays by males trying to attract
Sexual selection often provides an explanation for traits that are deleterious for survivorship,
because these traits increase the probability of finding a mate.
\Evolution Zol 445\2006\Sexual selection handout.doc