“The sight of the peacock’s tail makes me sick.” Quote of the Day “The sight of the peacock’s tail makes me sick.” Charles Darwin

Evolution and Sex

What’s so disgusting about the peacock?

Darwin’s Dilemma Many species have characteristics that seemingly should be eliminated by natural selection. Bright coloring and attention-getting behavior tend to attract predators. Large body size or highly exaggerated physical features require a large amount of energy to produce.

Darwin’s Solution Sexual Selection: the theory that competition for mates between individuals of the same sex drives the evolution of certain traits. Seemingly harmful traits can evolve when the sexual attractiveness of the trait outweighs the problem it presents for survival. Whereas natural selection is sometimes called the “survival of the fittest,” sexual selection can be thought of as the “survival of the sexiest.”

Causes of Sexual Selection: Male Competition Males have lots of sperm and often reproduce with several different females. Generally, males compete for the opportunity to mate with females.

Size Differences Larger, stronger, and tougher males are typically more successful in competitions for females and more likely to be able to defend their offspring. Over time, great size differences can evolve in species when males compete for access to females. Examples: baboons, lions, moose, crocodiles

Example: Elephant Seals Males (bulls): 4000-6000 lbs, up to 20 ft long Females (cows): 1300-2000 lbs, ~10 ft long

Causes of Sexual Selection: Female Choice Females have fewer eggs and invest more time and energy in reproduction. Generally, females hold out for the best genes and/or the most helpful mates.

Exaggerated Physical Features When females choose a trait that is closely linked to successful genes, that trait spreads throughout the population over generations. Sometimes these traits can become highly exaggerated, as seen in the peacock.

Exaggerated Physical Features in Males Lion Frigate bird Pheasant Elk

Comparison of Natural and Sexual Selection What is selected for? Who or what does the selecting? Natural Selection The best survivors Climate, Predators, Competitors, Disease Sexual Selection reproducers Members of the opposite sex

Runaway Sexual Selection When males with a specific trait mate with females that prefer that trait, the trait and the preference for it become genetically linked. This can lead to a "runaway" process, causing the male trait to become more and more extreme until the survival disadvantages of the trait outweigh the sexual attractiveness. This “runaway sexual selection” may eventually lead to extinction of a species. The Fate of the Irish Elk?