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Sunday, October 30, 2022

Peacock Tail

Thinking about the irrelevant criteria has helped me understand the classic arguments debating the purpose of the tail of a peacock.  One hypothesis is that peahens are attracted to the peacock with the largest tail because it signals that the peacock is genetically healthy.  The logic of this hypothesis appears to be that since possessing a large tail is an energy and maneuverability burden which can only be borne by the healthy, a larger tail is indicative of superior genetic health and therefore predicts better reproductive success.

While I think there is some truth to this, it does seems odd that a phenotype that is a liability to the survival of a father would be considered genetically advantageous by the mothers of the future sons.  I think there might be a mistake in the logical jump of assuming that the kind of health required for large tails is correlated with an overall health which leads to individual survival in general.  Another potential logical fallacy might be in assuming that the overall health and long-term individual survival of the father are correlated with more descendants for the peahens.

Suppose instead that the peahens are instinctively selecting the peacock with the largest tail because this decision is more likely to result in their future sons having larger tails which will then be more likely to attract future peahens which will then in turn be more likely to lead to both more grandsons and more granddaughters.  Since these peahens are most likely to mate with the peacock with the largest tail, the evolutionary advantage of winner-take-all breeding will rapidly lead to the great-grandsons having even larger tails than their great grandfathers and the great granddaughters being even more attracted to large tails than their great grandmothers.  The peacock tails keep getting larger as generations pass until the tails become so large that their reproductive advantage of attracting mates is balanced by their disadvantage of attracting predators.

At that point you might think that the size of the tails might stabilize.  Consider, however, that an undesirable side-effect of an otherwise beneficial gene might be neutralized by compensatory genes.  As the genes of peacocks to grow larger tails evolve, the genes to survive with larger tails also evolve to compensate.

Suppose that at one time proto-peahens were not attracted to larger peacock tails and that they mated non-preferentially with the males.  Assume that back then proto-peacocks had tails of whatever size was best adapted to their environment on average.  Now imagine that one day a proto-peahen is hatched with a mutant gene that gives her a slight preference to mate with proto-peacocks with larger tails.
You can see how this slight preference could kick off an irreversible runaway process.  Her sons are more likely to have the gene for larger tails because she mates preferentially with the largest-tailed male.  Her daughters are more likely to have the gene for largest tail preference because she passes that gene on to them.
Now wind back the clock on the thought experiment to the initial condition where proto-peahens have no preference.  Imagine this time instead that a mutant proto-peacock instead of a proto-peahen is born with a gene that gives the proto-peacock a slight preference for a larger tail in the opposite sex.  During a breeding season, the proto-peacock might mate initially with the proto-peahen with the largest tail but can then go on to mate with additional proto-peahens.

There is no winner-take-all effect here in that the male, unlike a female, can successfully mate and reproduce with multiple individuals of the opposite sex with both preferred and less preferred tail sizes within a single breeding season.  The mutant male is going to have daughters with both larger and smaller tail sizes in nearly the same proportion.  Even if the mutant father successfully passes on the new gene for a slight preference to the sons, the sons will still mate with both larger and smaller females which means the granddaughters will still have mixed tail sizes.
Even if the mutant gene for a preference makes it into the grandsons and there are now more females with larger tails due to initial preferential mating, any environmental disadvantage to larger tails could make the larger-tailed females less likely to survive to reproductive age or the next breeding cycle.  This means that both the mutant males that prefer larger-tailed females and non-mutant males with no preference will have more smaller-tailed daughters than larger-tailed daughters because there are proportionally more smaller-tailed mothers.  As there is little long-term reproductive advantage to mating preferentially with large-tailed females, the mutant gene to cause a slight preference in the males eventually dies out due to genetic drift.

This might explain why you are more likely to see phenotypic extremes that decrease the survivability of the individual evolve in male but not the female in many species.  An example of this is brightly colored feathers for males and camouflaged colors for females in some bird species.  The evolutionary advantage of fathering the chicks of most of the mothers in a territory outweighs the disadvantage of being more easily spotted or captured by predators.

For species where raising the offspring to maturity requires the full-time attention of both a dedicated mother and a father, it is less of a winner-take-all competition since males are less likely to fertilize multiple females within a breeding season.  In these cases, you might predict that the males of these species are less likely to exhibit phenotypic characteristics preferred by females that have a survival disadvantage for the individual that are not also present in the females of the species.  Examples of characteristics attractive to the opposite sex that are present in both sexes that are also survival disadvantages to the individual parent might include altruistic behaviors such as nurturing offspring or defending the pack.

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