Problem : Explain how large horns might evolve in territorial males through sexual selection.

Territorial males must fight with other males to maintain their territories, which give them access to females. If a male with large horns challenges a male with smaller horns, he will most likely win the territory and mate with the females, while the male with smaller horns will not be able to mate. In this way, alleles for large horns are passed down while alleles for small horns are not. Eventually, the frequency of large horn alleles will be very high and small horns will disappear from the population.

Problem : What type of sexual selection would cause large horns to evolve in territorial males?

The evolution of large horns would be caused by contests.

Problem : What is the difference between sexual selection by contest and by choice?

Selection by contest involves competition against other members of the same sex. Selection by choice involves selection for members of the opposite sex.

Problem : What type of selection causes showy, energetically expensive displays such as peacock's tails?

These displays are caused by selection through choice of one sex for members of the other sex. The peacock's tail evolved because peahens choose peacocks with showy tails over those with plain tails.