Darwin's Failed Predictions, Slide 8: "Why sexual selection?" (from JudgingPBS.com)
[Editor's Note: This is slide 8 in a series of 14 slides available at JudgingPBS.com, a new website featuring "Darwin's Failed Predictions," a response to PBS-NOVA's online materials for their "Judgment Day: Intelligent Design on Trial" documentary.]
According to PBS, the male peacock's beautifully-colored tail is easily explained using sexual selection: females prefer the colorful "eyes" on the tails of males. Has the evolutionary origin of the peacock's tail been explained?
Sexual selection merely pushes the question back: why should female peacocks prefer male peacocks with tails that have "eyes"? Absent a linkage to survival and reproduction, sexual selection is now a circular argument: male peacocks have beautiful tails because females prefer such tails, and females prefer such tails because they are, well, beautiful. Under sexual selection, explanations become arbitrary because traits are preferred simply because a biologist deems them "attractive." But sexual selection rarely provides an external adaptive reason to explain why such traits should be "beautiful" to the opposite sex.
Despite its ability to arbitrarily select for virtually any trait wished for, sexual selection has been invoked to account for the evolutionary origin of humanity's most cherished abilities, including art, literature, music, mathematics, religious belief, and even scientific genius. Once you define something as "beautiful" or "attractive," the magic wand of sexual selection can produce virtually anything an evolutionary biologist wants.
But there is a more fundamental problem here: the existence of sexual selection itself begs the very question, why are there male and female peacocks at all?, i.e. why did sexual reproduction evolve in the first place? Sexual organisms only pass on 50% of their genes to offspring, whereas asexual organisms make clones that contain 100% of the parent's DNA. Thus organisms that hypothetically evolved sexual selection suddenly experienced a 50% drop in fitness. The fitness cost of sexual reproduction is further explained in the critical response to the PBS Evolution series, Getting the Facts Straight:
"The very existence of sexual reproduction presents a problem for Darwin's theory. The easiest way for an organism to reproduce is simply to divide asexually-- to make a copy of itself. Bacteria are very successful at this. An organism that reproduces sexually, however, must divert precious energy into making sperm or egg cells; in the process, gene combinations that were quite useful beforehand are sometimes destroyed through 'recombination.' Then the organism must find a member of the opposite sex and mate with it successfully. From an evolutionary perspective, sex incurs considerable costs that must be offset by advantages to the organism."1The online materials for PBS-NOVA's "Judgment Day: Intelligent Design on Trial" documentary mentions none of these obstacles facing the origin of sexual reproduction.
1. Getting the Facts Straight: A Viewer's Guide to PBS's Evolution, page 73 (Discovery Institute Press, 2001), at http://www.reviewevolution.com/viewersGuide/viewersGuide.pdf.
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