Question

How the trait for the peacock tail can evolve trough predation and sexual selection?

Answer 1

An Introduction to Sexual Selection

Sexual selection is an important mechanism of natural selection which may not get enough attention in basic biology courses. The idea dates back to Darwin, who dedicated over half of one of his books to the subject. Sexual selection is based on the struggle to reproduce rather than the struggle to survive; this notion reinforces the different meaning of 'fitness' in evolutionary biology compared with its common usage. The concept of natural selection is often associated with some kind of struggle to survive. Some individuals in a population do better than others and so have more offspring; standard examples might be prey outrunning a predator (or vice versa) or bacteria developing resistance to an antibiotic. If the reason for their success is heritable (i.e., if it's something that is passed on to their children), then the children will also do better than average and the trait will gradually spread throughout the population. This is one straightforward interpretation of 'survival of the fittest', but restricting our understanding of natural selection to these sorts of cases would be an unfortunate oversimplification. In order for an organism to be successful, it has to do more than survive; it also has to find a mate and reproduce. Sexual selection is the result of individual differences in successfully attracting mates. It comprises two processes: mate choice, in which members of sex choose a mate based on displays of courtship signals; and mate competition, in which members of sex directly compete with each other for access to the other sex. In both cases, individuals have a higher fitness not because of their ability to survive, but because they are better at finding a mate and reproducing.

The classic example of mate choice is a peacock's tail. Peahens prefer peacocks with large and colorful tails, so those peacocks get to mate more frequently and have more offspring. The male peachicks will inherit the genes for a long and colorful tail from their father and so will have a similar tail. Peacocks with extravagant tails will have more children since they are more successful at attracting a mate; as a result, the peacocks in the next generation will, on average, have more extravagant tails. This is where things get interesting: since the tails become more and more impressive from one generation to the next, peacocks constantly need to do better and better to get the attention of a peahen. Each generation, the peahens prefer the peacocks with the most impressive tails, which leads to an increase in the average tail quality in the next generation, when the peahens will again select peacocks with the best tails, leading to an increase in quality, and so on. This positive feedback, which is a hallmark of sexual selection via mate choice, leads to a runaway process where a courtship signal becomes more and more extreme. The courtship signals used in mate choice are generally costly, either by requiring a great deal of energy or by making the individual more vulnerable to predation (or both). Costly signals are necessarily honest ones since an individual has to be in good health to be able to maintain them. In other words, size only matters if it's expensive or risky; if the signal is easy to fake, it isn't an honest indicator of good health or good genes. The costs also limit the runaway feedback process of sexual selection. When costs aren't a constraint, a trait might continue becoming more extreme until it hits physical limits.

The male peacock has a lot of showy feathers. One would think that these would not appear in evolution; they would be noticeable to predators and would get the peacock stuck so it would not be able to escape from predators or get food. However, peacocks use their feathers to attract peahens (female peacocks). The peacocks with the showier feathers are able to attract mates, so they are the ones that have offspring, and pass on the fancy-feather genes to the next generation. However, the organism must also be able to survive. Peacocks can fold up their tails, which lessens the danger of being noticed by predators or getting caught in the bushes. In addition, as with most species where the males are colorful or fancy to attract the females, the peahens are much duller and more camouflaged, in a large part because they are the ones who guard the eggs and chicks. Survival of the next generation is very important.

The peacock tail contains spectacular beauty because of the large feathers, bright, iridescent colors and intricate patterns. The colors in the tail feathers are produced by an optical effect called thin-film interference. The eye pattern has a high degree of brightness and precision because the color-producing mechanisms contain an extremely high level of optimum design. According to the theory of sexual selection, the peacock tail has gradually evolved because the peahen selects beautiful males for mating. However, there is no satisfactory explanation of how the sexual selection cycle can start or why the peahen should prefer beautiful features. In addition, there is irreducible complexity in both the physical structure of the feather and in the beautiful patterns. Most birds have two types of tail feather: flight feathers and tail-coverts. The flight feathers provide stability during flight, while the tail-coverts ‘cover’ and protect the tail region. In the vast majority of birds, the tail-coverts are small feathers, just a few centimeters long. However, some birds like the peacock have very large tail-coverts for decorative purposes. These decorative feathers are also referred to as ornamental feathers or display feathers.1 It should be noted that a peacock is a male peafowl and a peahen is a female peafowl. The peahen does not have any decorative feathers. When a peacock displays his tail feathers during courtship, a magnificent ‘fan formation’ of feathers forms a beautiful backdrop to the body of the peacock as shown in Figure 1 (below). An adult peacock has an average of 200 tail feathers and these are shed and re-grown annually. Of the 200 or so feathers, about 170 are ‘eye’ feathers and 30 are ‘T’ feathers. The ‘eyes’ are sometimes referred to as oscillations.