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On Sexual Selection
Topic Started: Nov 28 2017, 02:36 PM (346 Views)
Sheather
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I recently read an interesting book by Richard O. Prum, The Evolution of Beauty: How Darwin's Forgotten Theory of Mate Choice Shapes the Animal World - and Us. The book is a little technical but not overly so and explores various aspects of sexual selection in different animals, mostly birds, and how sexual selection acts completely independently of natural selection in regards to producing animals fit for survival. In this work he notes birds such as the club-winged manakin, which has enormous, swollen wing bones that it claps together to produce a courtship song. For some reason females find this extremely erotic, and mate with the males with the most distended wings. The side effect of this, however, is that the wing bones begin developing in the embryos of this species before the sex of the bird is established, so that females have inadvertently selected for themselves to also have these ungainly, inefficient wing bones that make them poor, labored flyers and demonstrably reduce their fitness. However, the females' wing bone is hollow, whereas the male's is solid, apparently because the pneumatization of the bone occurs later in development - and so, the female wing is not quite as ungainly as the males, though still more so than a typical wing.

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a comparison of the ulna of a club-winged manakin (male center, female right) with the ulna of a related white-crowned manakin, showing the grossly enlarged structure of the bone that, while attractive to the female, impedes flight efficiency.


The author goes on to suggest that the manakin has found itself in a feedback loop of selection for decreasingly fit traits that make life more difficult for all members of the species, entirely due to female preference for the large wing bones that produce the male's vocal courtship song. While natural selection, in particular predator pressure, still operates and will weed out the most dramatically inefficient individuals which can't fly at all, it is now engaged in an arms race with sexual selection in the form of female mate choice, creating a species that teeters on a thin line of how inefficient can you become without being totally unable to survive. Another example is a certain species of bird of paradise with a bright blue bald head. Females love the trait in males, and mate with those with the brightest cap; inadvertently, because feather distribution occurs before the developing embryo has a sex, they have also selected for themselves to have the same trait, which is totally useless and indeed, deleterious, as it makes them more obvious when sitting on their eggs (however, their cap is slightly less vibrant, since pigmentation can be changed after the sex of the bird develops.)


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A club-winged manakin performing its wing song.


The author also hypothesizes that such other male attributes as ornate plumage and other supposed handicap traits, which were long suggested to exist as a way to show a male's fitness (the classic example of "I can carry around this long tail and still not be eaten by a tiger, mate with me for strong babies") may actually fall flat, and that females may simply prefer aesthetically pleasing mates, as a great many male traits are not actually maladaptive, such as plumage that is normally hidden from sight and so does not make them more visible to predators. Even the large tail of a peacock, the archetypical example of a handicap trait, is actually so lightweight as to hardly make any difference in the ability of a peacock to escape danger (and is easily shed if grabbed by a predator, too, thus not really reducing the peacock's fitness at all.) The author suggests that the majority of supposed handicap traits are actually about useless to broadcast the fitness of the animal bearing them, except to say that they are not literally on the verge of death, because even animals with defects, disease, and on poor diets can still produce the normal traits in many conditions. In fact, less fit birds may actually produce better plumage, as the genes for it are not necessarily correlated at all with overall vigor or the lack of genetic disease, meaning that the value of these traits, and the reason they persist, may actually be nothing more than the fact that females like them. Alternatively, they could have initially developed as a valid way of determining a male's fitness but then degraded into nothing more than aesthetic choice, in which less fit males with prettier looks can effectively trick partners into thinking they are better mates than they really are. An example in the book is that the feather displays of some birds may have originated as ways to allow a female to inspect her mate for feather mites, but as the plumage became more showy, the females simply developed a preference for that alone and indeed in many birds it is now so extreme as to surely not be evolved for any practical purpose alone.

Indeed, this segues onto another theme discussed in the book, that males and females of a given species often have very different agendas, and that not only do species as a whole have to fight over the eons to adapt and survive outside forces in natural selection, but they are often in competition within their species. A good example here is of ducks. A female duck wants to be able to choose which male duck fathers her little baby ducks, and so over many millions of years the drab little female ducks, who have to be brown and plain in order to hide whilst incubating their eggs, have selected for very pretty males with beautiful plumage, who all vie for her attention. Because feather color is determined long after the sexual differentiation of the embryo, a hen duck selecting for colorful mates will not inadvertently also select for colorful hens as has occurred in the wings of the manakins, and so she does not run the risk of producing less fit female descendants. But as she chooses only the most aesthetically pleasing drakes, she does create another issue both for herself and her descendants, in that the more extreme and beautiful males she cultivates through her mate choice, the more males will exist which don't live up to the expectations of of females of her species in the future.

If the females of their species will only choose to mate with the most beautiful males, then those that are not chosen would die out, right? Well, this would be the case if the male ducks accepted their fate, but they don't, and this is where the males of the species begin to actively compete with the females as to who's genes survive by attempting to totally overrule their ability to choose their mates and rape the females against their volition. The aggressive forced copulation of ducks is well-documented, and is believed to have evolved as a direct result of female mate choice. If the female of the species is going to be picky and only mate with the most perfect partner, then all those other males that don't fit the bill have had to adopt an alternate way of producing offspring, and they do it with force. Forced copulation does produce more offspring from unattractive male ducks than they would otherwise produce (which is none), but it is a very bad situation for the female, as she often drowned by gangs of aggressive drakes in the process (and thus produces on the whole far fewer young than she would with a single sexy mate of her own choosing.)

It is so that multiple distinct types of male ducks have been selected for; the one the female duck prefers is beautiful and gentle, but most male ducks are not like this, and so if the hens had their way only a tiny fraction of available male ducks would ever mate. For every beautiful, gentle male there are probably a number of less attractive ones, which never mate because they aren't pretty enough to be chosen or aggressive enough to force themselves on females, so these don't spread any genes at all. The second group which does are the "ugly" and aggressive males that the females like least, but which don't care and force themselves on anything that moves with no regard for the female's well-being. Females avoid these males at all costs, but they often work in groups to overpower the females, and so they are still reproductively successful. The female duck has thus created a problem to which there really isn't a solution by becoming so choosy about the appearance of her mate, because the males she would never otherwise choose have developed an alternative, and for the female harmful, behavior to circumvent female mate choice. Even if a female chooses the handsome, gentle partner, she is still likely to also be assaulted by the mean, ugly one, and so produce ducklings which will grow up to exhibit both variations in appearance and behavior, continuing the battle of the sexes into the foreseeable future.

Mr. Prum also goes into detail about sexual selection in the great apes, particularly the common ancestor of humans, chimpanzees and bonobos. Modern chimpanzees exhibit a reproductive situation where females accept virtually every male that comes by when in their fertile period, and it is believed they do so to trick all of the local males into thinking they are the father of her baby in the future, because a male will kill any babies he believes were not his own, and do so remarkably often, to the point where attacks by adult males are one of the highest causes of infant chimp fatality in the wild. Males still fight over mating rights, even though the female will accept all of them, because they don't want other males siring babies that could be theirs, and so even if the female lets all of the males mount her, there is still unfortunately a good chance that one or more will try to kill her offspring if they believe it belongs to another male, and so the situation really has no perfect solution, and the author suggests it is quite terrible to be a female chimpanzee.

Bonobos have developed a different strategy to avoid male infanticide, and it involves lots of sex. Rather than fight, males have sex with each other, turning competitors into partners, reducing violence in the group, and reducing pressure on the female to the point that infanticide in the bonobo is virtually unheard of. Because the males are much less aggressive (largely because they're busy mating with each other), the female has more freedom to choose her own partner out of all those available without worrying about those she does not choose killing her baby. Bonobo society is thus much more pleasant than chimpanzee society, and females have a lot more control over their reproductive choices. This is a stark alternative to the duck situation, but has arisen in a similar way.

The author also hypothesizes the diversity of human sexuality has evolved in a similar way. To avoid infanticide, as well as to provide females greater ability to choose their mates rather than have intercourse forced upon them, the female ancestors of humans selected for male homosexual behavior - not exclusive homosexuality, but the frequency of male on male intercourse, which as in the case of bonobos would reduce pressure on the females in a group as these males would be less inclined to fight each other over mating rights with females and more likely to just have sex with each other. This would result in stronger male bonds, lesser frequency of forced male on female mating and reduced infanticidal tendencies on the part of males. As in bonobos, if the males were otherwise more often occupied by each other, it was also gave the females a much greater ability to choose which of them the did want to mate with without the others becoming jealous or aggressive, thereby giving the female sex an upper hand in choosing which genes were passed on to future generations. Male sexual behavior, conversely, is suggested to have given rise to female homosexual behavior, as it would increase female bonding to produce strong, female bands better able to drive off aggressive raping males. The author thus suggests that male and female sexual variation has developed not by a single means but at least two opposing forces, both directly tied to mate choice from the opposite sex.

Over all, sexual selection seems to be something not often touched upon in speculative evolution, which seems to mostly follow only natural selection. In many ways, it's kind of like artificial selection done by humans, except the selection occurs over a very long period of time through the mate choices of individual members of the species in question - in effect, they modify themselves. If sexual selection is considered as an additional driving force to evolution, it means that the evolution of life is really even more complex than we've long anticipated. One sex can over time change the appearance and behavior of the opposite sex dramatically through their mate choices, but sometimes this can backfire on them, like with the ducks. Males and females often want different things, driving competition even within a single species that can have very dramatic effects, sometimes even negative ones to overall fitness (such as females exhibiting useless male sexual attributes.) Females, in their quest for beauty, can inadvertently gift themselves with handicap traits that while not necessarily harmful to the males are harmful to themselves, particularly for female birds which might love colorful traits but who ideally should have none of their own lest they be too visible to predators whilst nesting.

It's notable that on Earth, almost all aesthetic mate choice is done by the female, with the male being the one to develop the "handicap' traits like antlers and peacock tails and the females staying plain. In creature design respect, I can imagine speculative animals in which the males, too, have their own ideas of beauty that might be totally different from the female ideas of who makes a good mate, producing extreme sexual dimorphism, such as a tiny flying male and a sedentary female, or the reverse, or depending on the way they raise their young perhaps both sexes are extremely showy but in totally unique ways. Natural selection alone is unlikely to produce extreme sexual dimorphism (the most it does on Earth is to keep female birds plain), but once you factor in sexual selection, it would be possible to produce some very unique organisms unlike anything on our planet. I think it would be an interesting thing to take into account in our projects, anyway!
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