Sexual Selection

Read the information below (and then use additional resources as needed) to complete your Sexual Selection side quest handout.

Natural selection clearly explains why there are differences between species, but it isn’t enough to explain the often remarkable differences between males and females of the same species. Some examples of such differences include:

Longer manes in male lions and beards in male humans
Antlers or horns in male deer and cattle
Bright and extravagant plumage of birds

Darwin proposed that these characteristics were the result of a secondary mechanism operating alongside natural selection - a mechanism he called 'sexual selection'. In evolution, having traits that help an organism survive is very important, but only to the extent that they are able to reproduce and pass their traits (genes) onto the the next generation. For this reason, traits that help an organism successfully reproduce are selected for, even if the traits don’t necessarily help them survive. Of course, there are limitations to the traits that can be selected for because natural selection is still at play. If, however, the selective pressures for survival are minimized (e.g., lots of food and few predators), sexual selection can produce extravagant traits. Although sexual and natural selection may seem different, they are actually driven by the same basic process: competition to leave more copies of your genes in the next generation! Below are descriptions of the two methods that drive sexual selection: female choice and male-male competition.

Female Choice

When a primary food source is spread out (“dispersed”) and impossible to defend, males need to try to attract females and convince them that they are the best possible mates. Since females tend to be attracted to fancy or flamboyant males, the males tend to become larger, more brightly colored, or they show off with extravagant courtship displays (mating rituals). These “fancy” traits might start small, but with each generation the males become more and more elaborate; that is until those traits significantly decrease their chances of survival. Females, on the other hand, don’t change because there aren’t any selective pressures driving them to evolve.

Male-Male Competition

When a primary food source is clustered together, males tend to create and defend territories. These territories not only provide males with a steady food source, they give them exclusive access to the females that go there to eat. Males that are bigger or stronger are better able to defend their territory and mate with more females. Consequently, the bigger/stronger males will pass on more of their genes to the next generation. Each generation of males will get burlier and burlier until the traits affect their survival. Once again, the females don’t change because there is no advantage for them to carry the extra weight.

Watch this video to help learn more about sexual selection in birds.

Additional Resources

Lessons in Seduction from the Animal Kingdom

Use the embedded link to learn more about how males from a variety of different species try to attract mates.

Fancy Males

Can you identify which bird is male? Test your skills using the embedded link above.

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