The Peacock Problem
‘The sight of a feather in a peacock’s tail, whenever I gaze at it, makes me sick!’ So wrote Charles Darwin in a letter to his friend, expressing his frustration at not being able to explain how natural selection could drive the evolution of this extravagantly ornamental display. Not only was there an obvious lack of survival advantage to an awkwardly heavy appendage, it came with an energy cost and added vulnerability to predators. How then, did the peacock’s tail evolve?
Once again, it was Darwin who came up with the idea of sexual selection, that depends, “not on a struggle for existence, but on a struggle between the males for possession of the females; the result is not death to the unsuccessful competitor, but few or no offspring”.
By flaunting his “handicap”, the peacock signals to his potential mate that he has survived despite the negative consequences! The good gene hypothesis suggests that the ornament is a proxy for a healthy immune system and metabolic fitness. The peahen’s preference for gaudy displays drives the evolution of the tail by positive feedback: when she mates with the most fashionable male, she passes his traits on to her sons who in turn, are assured of reproductive success! Choosy mothers produce sexy sons and over many generations, runaway evolution results in strange and beautiful ornamentations like the lion’s mane, the antlers of a stag and the blue-footed booby. In the 20th century, Ronald Fisher, who is considered the greatest evolutionary biologist after Darwin, argued that the female’s preference and the male’s development of the ornament must advance together until practical or physical limits halt any further exaggeration (http://en.wikipedia.org/wiki/Fisherian_runaway).
We’ve seen how sexual selection gives rise to the difference in appearance between male and female (sexual dimorphism). Animals that are monogamous show less sexual dimorphism. Interestingly, our pre-Homo ancestors may have been more dimorphic compared to modern humans suggesting that we have become more monogamous over time!
REF:The sight of the peacock’s tail makes me sick: the early arguments on sexual selection. (2000) Hiraiwa-Hasegawa M. http://www.ncbi.nlm.nih.gov/pubmed/10824193