The peacock’s tail presented a problem for Charles Darwin. It clearly could not have arisen by natural selection. Its beauty, extravagance, and most of all, its impracticality seemed to argue against the gradual increase, generation after generation, of such a structure within a population.
Darwin introduced a separate process, sexual selection, to explain the origin of traits whose only apparent function is to influence the choice of mates. This process, like natural selection, would produce more offspring with the trait in question but would do so without reference to adaptation to the surrounding environment. In some cases, Darwin argued, sexual selection could act in opposition to natural selection to produce impractical results. I question whether such a process is plausible or even necessary to explain exaggerated traits.
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There is one way in which mating displays can evolve through natural selection. If they are providing reliable information about which mate is better adapted to the environment, then the displays will increase in the population through linkage to the adapted traits. An example in birds is the correlation between bright plumage and healthy immune systems. Of course, it helps if the selecting gender – often the girls- notice the mating display and are discerning about who they mate with. The mix of adaptive and mating traits are passed on after adjusting for the inconvenience and cost of the mating display. That is, peacocks with really heavy or awkward tails get selected out, while those that balance the needs of feeding, predator avoidance and attracting mates pass on their genes. If the mating display no longer indicates a quality mate – when the display is all style and no substance- natural selection will tend to reward less attractive individuals who nonetheless yield a lot of offspring.
Sexy sons
There is an idea that sexual selection occurs when females mate with attractive males for the prospect of having attractive (and potentially reproductively successful) sons. This is called the sexy son hypothesis. This amounts to the continuation of a fad over many generations. Given the fact that human fashions and fads last a small fraction of a generation, it seems unlikely that such a process could be important in evolution. Ronald Fisher, a brilliant statistician and evolutionary biologist, came up with a possible explanation of what he called “runaway selection”. The mating display is linked, not with a useful trait for survival as in the example above, but to a genetic preference for the display when choosing a mate. This could happen if 1) the genes for the mating display and the genes that influence the choice of that display occur close together on the same chromosome and tend to get inherited together and 2) the mating display doesn’t pose a disadvantage that natural selection would weed out. The offspring of a male with an exaggerated display(e.g. larger antlers, longer nose or brighter tail) and a female with a genetic disposition to choose that display will reinforce the frequency of the trait and its selection in the next generation. These two traits have to arise spontaneously at about the same time or else the process will not work. It is possible to have a display chosen without any genetic preference for a few generations simply on the basis of choosing familiar looking males. However, novelty and other survival characteristics will soon crowd out such a meaningless display unless the gene for choosing such a mate also appears in the population. To me, it is implausible that two genes will appear close together on a chromosome at about the same time having to do with the same structure. If such a thing does occur it must be very rare.
Signal amplification
There is a simpler way that mating displays can arise. If a small population is reproductively isolated (see Limits of natural selection ) then there is potential for new mating displays to arise by genetic drift alone. Still, it seems odd that genetic drift (literally, random change in the frequency of traits) would yield such extreme structures. If a small group of males are competing to attract mates it makes sense that the ones with the most extravagant display will have the biggest impact on the females, regardless of what their genetic preferences are. In communication, a clear, strong signal is needed to stand out from a background of competing signals. In this way, genetic drift can be drawn towards extreme (large, colorful or elaborate) mating structures in small populations without adding any assumptions of linked genes or hidden function.
In the end, it is possible that neither natural or sexual selection is involved in the origin of the peacock’s tail. In fact, if Darwin had seen the potential for reproductive isolation to establish new species, the issue may never have come up.
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