Sexual selection and how the peacock got its tail

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By David Annis
Article ID: 1242

The tail of the peacock is a magnificent and beautiful thing to behold.  In a previous article, I explained how complex structures (like eyes) evolve through a series of small steps, each of which gives the animal better vision. But how do ornamental things evolve, like the tail of a peacock?  The answer is sexual selection.

Charles Darwin actually proposed the concept of sexual selection in his book “On the Origin of the Species” in 1859.  Evolution is caused by the competition to leave offspring. Genes of individuals that have random variations that make them more likely to be able to successfully leave offspring will be passed on to future generations, and the characteristics of those individuals will be inherited by future generations.  One way in which individuals compete to leave offspring is by competing for mates: individuals that mate more successfully are more reproductively fit than their counterparts.

Sexual selection can take many forms. One form is combat, where an animal can fight with potential rivals.  For example, take two male bettas (also known as Siamese fighting fish). Place these in an aquarium. The male bettas will fight viciously with one another.  Battles for mates sometimes lead to the evolution of features that are used in the fights, like antlers of deer and moose. Sexual selection can also be “cryptic”. This means that a female can increase the odds of fertilization by the sperm of desirable males and decrease the odds of fertilization from less desirable males.

In many animals, sexual selection takes the form of elaborate ornamentation – often in just the male of the species.  Two well-known examples are a peacock’s feathers and the brilliant red plumage of the male cardinal.  Scientists like Robert Trivers have theorized that in most species it is the male that competes for female access because the female makes a larger contribution to the offspring.  In one experiment, scientists were able to vary the relative contributions to offspring in Katydids and they observed that when the males’ contribution was more valuable, females began to compete for males.

However, there is significant cost to these ornaments. So why would the genes that direct the growth of ornamental structures not reduce reproductive fitness?  After all, the energy that a peacock uses to produce his tail could be used to find a mate or produce sperm.  In this example, there are two hypotheses about sexual selection. One is that the ornament is a signal that the male is healthy. After all, a sickly male can’t produce and maintain the elaborate ornamentation.  The second theory is that females select males with elaborate ornament, because doing so ensures that their sons will be “sexy” and have a lot of mating opportunities; it becomes a sort of self perpetuating selection mechanism.

Sexual selection can be measured experimentally by exaggerating a sexually-selected characteristic.  Methods that scientists have used to study sexual selection include lengthening and shortening long ornamental tails in swallows, and painting spots on butterflies.

Even in monogamous species sexual selection can play a role in a single breeding season.  An example comes from DeLope and Moller, who lengthened and shortened the tails of male swallows during a breeding season and found that the females adjusted their reproductive efforts in response (including the clutch size and number of clutches in a season).

So, the next time that a creationist claims that the great beauty found in nature can’t be explained by evolution, remember just how sexy beauty can be.

Other articles related to this topic:

• Evolution makes testable predictions
• A review of “On the Origin of Species” by Charles Darwin
• What good is half an eye?
• Five atheist logic tests and how to pass them – a skeptical response to “How to make an atheist backslide”
• Macro-evolution observed in the laboratory