Shake a tail feather
Sexual Selection in Peacocks
"The sight of a feather in a peacock's tail, whenever I gaze at it, makes me sick."
That is what Charles Darwin wrote when first thinking about the showy and elaborate tail feathers of the male peacock. If traits that aided in survival were selected for and were more likely to be passed on to the next generation, how was it that evolution by natural selection could produce something like the brightly colored and elaborate tail feathers of a peacock? How could such traits evolve when they are more noticeable to predators and make fleeing from them more difficult? Clearly the tail feathers of the peacock were a problem for his idea of natural selection. Or were they?
Do flashier tails really attract more mates?
Darwin began to wonder if there was more to his idea of selection then just traits being selected due to increasing an organism’s chances of survival. To pass on traits to the next generation, a peacock needs to survive AND reproduce. If a trait more successfully attracts mates and increases the number of offspring that an individual produces, that trait will be passed on to the next generation. That idea is one that Darwin eventually added to his idea of natural selection and it came to be known as sexual selection. If more elaborate tail feathers attract more peahens, then this sexual selection could explain the evolution of such a flashy ornament for peacocks.
Darwin never had the data to test his idea of sexual selection in peacocks, but a now classic study done in 1991 did just that. In the study, researchers counted the number of total eyespots on the tail feathers of individual peacocks. They then observed free-living peacocks and counted the number of peahens that each individual mated with.
Here's the activity:
1) Start by asking your students to make a graph with these data to see if the researchers’ prediction that peacocks with more total eyespots on their tail feathers will get more mates is supported or not.
2) Do these data support Darwin’s idea of sexual selection?
These data come from:
Petrie, Marion, Halliday Tim, and Sanders Carolyn. "Peahens prefer peacocks with elaborate trains." Animal Behaviour 41.2 (1991): 323-331. Link to paper as PDF.