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Author Topic:   The limitations of Sexual Selection
Dr Adequate
Member (Idle past 284 days)
Posts: 16113
Joined: 07-20-2006


(1)
Message 6 of 36 (620232)
06-14-2011 11:12 PM
Reply to: Message 1 by slevesque
06-14-2011 2:23 PM


Because the very fact that a female prefers bright colors over dull colors is itself a trait of the female, and is therefore also subject to natural selection. Doesn't this imply that sexual selection is limited by natural selection, and therefore have explanatory power only in the complete absence of predators ? (which is a rare occasion)
How would a female population aquire the tendency to prefer bright colors in an environment where predation is present ? Wouldn't natural selection work against this ?
It is fairly easy to see why conspicuous secondary sexual characteristics persist once they exist. They are part of an ESS --- an evolutionarily stable strategy --- which may be defined as a strategy which is optimal to follow if everyone else is following it.
Consider the conspicuous plumage of the peacock. A male with drabber plumage would be more likely to escape predation, and would be favored by natural selection (in the narrow sense, i.e. excluding sexual selection), but because females make sexual choices on the basis of plumage, he would be disfavored by sexual selection.
But why, you might ask, can't the species evolve so that peahens prefer drabber plumage? Well, this is where the notion of an ESS comes in. Such a preference would have to start with changes to the genes of one peahen. She would mate with the peacock with the drabbest plumage. Her sons would be more likely to survive, but they would be selected against by sexual selection because all the other peahens would still be selecting for conspicuous plumage.
We'd therefore expect the balance of conspicuousness to camouflage to be around an equilibrium such that any more or less conspicuousness would be penalized by, respectively, the decreased chances of survival outweighing the increased chances of reproductive success, and the decreased chance of reproductive success outweighing the increased chances of survival. Natural selection pulls one way, sexual selection pulls the other way, and "the complete absence of predators" is not (as you suggested) necessary for sexual selection to make the males of a species somewhat conspicuous, any more that the complete absence of female choice would be necessary for natural selection to make them somewhat drab.
The only way natural selection could win out completely is if the predatory pressures were so great that only drab peacocks would survive. If this situation came about suddenly then it is more likely that the species would be driven to extinction.
So generally speaking once a species has conspicuous secondary sexual characteristics it's stuck with them.
* * *
I think that answers your first question. The second question is, I think, more interesting: how would such a state of affairs arise in the first place? Note that what follows is my own hypothesis --- I don't mean that it definitely hasn't also been developed by others (and I would suspect that it has, because scientists aren't all idiots) but I do mean that I have no positive knowledge that it has been submitted to the scrutiny and criticism of the wider scientific community. I feel, however, that it is so luminously obvious that you might be willing to overlook this.
I think the key to understanding the evolution of conspicuous secondary sexual characteristics is the concept of a supernormal stimulus. You will probably be familiar with the classic investigation into juvenile herring gulls begging their mothers for food. They identify the maternal beak by very simple cues; it is possible to construct a fake beak which presents these cues and elicits this behavior even more strongly than a real beak, so that presented with a fake beak and a real female herring gull waiting to feed them the chicks will concentrate all their attention on the fake beak and slowly starve to death.
Now, similar things happen with mate selection. For example, consider the long-tailed widowbird. The female has a ~7cm tail, the male ~50cm. When researchers artificially extend the tail by sticking feathers to it, so that the tail is much larger than that found in nature, birds so treated become even more attractive to females, rather than being disdained as freaks. (Andersson: Female choice selects for extreme tail length in a widowbird, Nature, 1982.)
I read of a fascinating study for which I am temporarily unable to find a reference. A computer simulation was made of the visual cortex of a bird (again, of a species with a female preference for long tails) to see what was going on in there. What I found interesting was that there was not one bit of the brain to detect male birds and another to find how sexy they were; instead, long-tailed birds looked more like male birds; they stimulated the "male bird detector" more strongly; and an image of a supernormally long-tailed bird looks more like a male bird than an actual male bird does.
Now all this suggests how extravagant displays of secondary sexual characteristics get started. Suppose that a female bird can recognize males of her species by a wing pattern as in pattern A in the diagram below, which might be part of the male's camouflage plumage. Then she will probably be even more strongly attracted to plumage patterns B through E, which look even more like a male bird's plumage than the actual male bird's plumage does by virtue of being crisper and brighter and more colorful and more contrasting.
So long as the reproductive rewards of being conspicuously male outweigh the hazards associated with being conspicuous, the net effect of selection will be to drive the species towards the more conspicuous state.
Now of course natural selection can't look ahead and say: "My gosh, this mechanism for females to recognize males can be subverted in a way that will ultimately be bad for the species as a whole, since it will place a premium on being conspicuous to predators, so I'd better come up with another recognition mechanism which can't be subverted in this way"; natural selection never looks ahead. Given only the small brain of a bird or a fish or the smaller brain of a butterfly in which to instantiate a mechanism for identifying potential mates, it would not surprising that this subversion of mate recognition systems should happen quite often.
(I believe that a similar mechanism applies to aposematism. I have noted that the markings on aposematic species often strongly resemble in shape the camouflage markings of closely related non-toxic species. However, I should admit that (a) my researches into this phenomenon have not been terribly systematic and so may be affected by confirmation bias and (b) this might be the result of developmental constraints. This said, a similar mechanism is at least plausible: once a bird has learned to avoid a beetle with certain markings originally evolved for the purposes of camouflage, it would be still more inclined to avoid beetles in which markings of the same form were clearer and more distinct.)
* * *
The theory laid out in this post leads to various predictions. If a species has secondary sexual characteristics which make it conspicuous to predators (and if they aren't doubling up as aposematic signals) then we should generally find that:
(1) Artificially enhancing these characteristics will make their possessor more attractive to the opposite sex; artificially decreasing these characteristics will make it less attractive. Behavioral ecologists have performed innumerable experiments showing this to be the case, of which Andersson's work on widowbirds may stand as a typical example.
(2) Artificially decreasing these characteristics will make it safer from predators, and increasing them will make it more vulnerable. I can't remember reading about an experiment to test this proposition, possibly because it's hard to get funding to do research into the bleedin' obvious; naturally an organism which is easier for predators to find is more likely to be eaten by them.
There may be other costs associated with such characteristics; for example, I suppose that it is more energetically costly to drag a big tail around. (There would also be a cost associated with growing it in the first place, though one couldn't investigate this experimentally.)
(3) Increasing or decreasing the threat from predators should shift the equilibrium between natural and sexual selection and so cause the species to become respectively less and more conspicuous, as in the experiments with guppies that you reference in your post.
(4) If we can find out how a female of a species recognizes a male, and if females exercise choice between potential mates (which is usually though nor invariably the case), then it should be rare for the recognition mechanism not to be subverted by an exaggeration of this characteristic beyond what is necessary for the female merely to recognize that a male of her species is indeed a male of her species. This is somewhat of a negative prediction: but certainly I've never read (for example) that the female meadow-pipit identifies potential mates by a tiny, almost invisible dark-brown on medium-brown dot on the wing of the male meadow-pipit and is entirely unmoved by an increase in the size of the dot or the number of dots or the degree of visual contrast. And if I'm right, it is extremely unlikely that I will ever do so.
Edited by Dr Adequate, : I gave the wrong date for Andersson's paper.

This message is a reply to:
 Message 1 by slevesque, posted 06-14-2011 2:23 PM slevesque has replied

Replies to this message:
 Message 13 by slevesque, posted 06-15-2011 1:26 AM Dr Adequate has replied

  
Dr Adequate
Member (Idle past 284 days)
Posts: 16113
Joined: 07-20-2006


Message 14 of 36 (620259)
06-15-2011 1:27 AM
Reply to: Message 9 by slevesque
06-15-2011 1:15 AM


Any references for this ? I have a hard time thinking this is true.
Googling on "dull plumage" "sick birds" reveals that dull plumage can be a symptom of bacterial infection, fungal infection, malnutrition, and in short practically everything that can go wrong with a bird.

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Dr Adequate
Member (Idle past 284 days)
Posts: 16113
Joined: 07-20-2006


Message 16 of 36 (620263)
06-15-2011 1:44 AM
Reply to: Message 8 by slevesque
06-15-2011 1:14 AM


But if the trait is not fixed, and there is a range of preferences within the female population, how can it form anything else then at best an unstable equilibrium, one in which natural selection will in the long run always fix the most 'fitness-friendly' preferences in the females.
But experiments consistently show that the females want more of the maleness-indicating trait than males actually possess. To refer to the widowbird experiment again: the females like long tails, and so you can make male birds more attractive by making their tails longer than they are in nature.
Now the first female to decide that maybe 40cm would be a better length for a male widowbird's tail would get clobbered by the ESS, since even if she could find such a male, no other female widowbird would find her sons attractive.
Hence the upper limiting factor on how long the tails should be is provided by natural selection, not female choice.
Such responses to supernormal stimuli are common in birds. For example, they prefer looking after large eggs rather than small ones. Give them a fake egg larger and more brightly colored than a real egg, and they will ignore the real egg to sit on that --- even if it's so big that they keep sliding off it.
Edited by Dr Adequate, : No reason given.

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Dr Adequate
Member (Idle past 284 days)
Posts: 16113
Joined: 07-20-2006


Message 17 of 36 (620264)
06-15-2011 1:50 AM
Reply to: Message 15 by slevesque
06-15-2011 1:32 AM


I know it has been hypothesised, but I see nowhere where it was demonstrated, and I see no genetic reasons for why traits for color should be linked with overall health.
It's not that being drab makes birds susceptible to illnesses, it's that being ill makes birds drabber. So a bird which was drab as a result of a mutation would look like it was sick even though it wasn't. This would give the impression that it was poor husband material and indeed not a good bird to hang out with generally in case whatever was wrong with it was catching.

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Dr Adequate
Member (Idle past 284 days)
Posts: 16113
Joined: 07-20-2006


Message 20 of 36 (620275)
06-15-2011 4:19 AM
Reply to: Message 13 by slevesque
06-15-2011 1:26 AM


I can see perfectly see how an equilibrium would be maintained once the prefenrece is widespread in the female population. I'm more concerned on how such a situation could come to such a point in the first place, and how even in that case it can remain a stable equilibrium in the long run since natural selection would not only select for the trait itself, but also for the traits of the female preferences also.
I think I've explained all that by now, but get back to me if you have any questions.
What about my corrollary in my OP ? What if we took a population of guppies, for example, in which females select for conspicuous colors, and put this population under predatory pressures. Will there come a point where not only will the camouflage trait of males will become fixed (or nearly fixed, this happens rather quickly as per the Endler experiments) but also the female preferences for camouflage colors also become fixed within the female population ?
Another interesting question.
I would hesitate to be dogmatic but I suspect the answer is "no".
First of all, we have no reason to think that females would develop a preference for camouflaged males as such. And there is an argument against it: a female who thought the sexiest males were those which most resembled the background would necessarily think that the background itself was sexier than all the males, with unfortunate consequences. The females must surely identify potential mates by those characteristics which distinguish them from the background.
This is a general consideration, but there are also reasons for doubting it that arise from thinking about guppies in particular.
In the case of guppies we know what happens in the absence of predation --- male guppies develop finer spots on a coarser background and coarser spots on a finer background. In my previous posts I have made much of the concept of being conspicuously male, but there is also an advantage to suitors that are conspicuous per se. Without any considerations of the workings of a female guppy's brain, we can see that there is no chance of her wanting to mate with a male whom she doesn't even notice; and other things being equal, what hides the male guppy from predators will also hide him from potential mates. It has been said that the first rule of success is "show up", and in the case of male guppies this may be taken in a double sense.
Finally I have to wonder if even under strong predation the male guppy ever does attain the same level of crypsis as the female guppy. We know that under the pressure of predation the spots of the male guppy get larger or smaller, but is the conspicuous sexual difference ever entirely eliminated?
Here's a picture of a couple of guppies, so we can all see what we're talking about. The female is the larger of the two, as is common in fish. It is probable that these are aquarium-bred specimens.
Bonus fact: the guppy was discovered by a man called Robert John Lechmere Guppy --- hence the name.
Edited by Dr Adequate, : No reason given.

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Dr Adequate
Member (Idle past 284 days)
Posts: 16113
Joined: 07-20-2006


Message 21 of 36 (620280)
06-15-2011 6:35 AM


Looking for information about supernormal stimuli in fish, I found this brief account of his own work by James Gould:
My approach has been to look at female preferences in species without female choice, to see if (when allowed to choose) females have biases in the absence of the opportunity to express them. I have concentrated on a family of live-bearing fish which includes mollies, guppies, mosquitofish, platys, and swordtails. I find that females in male-contest or male-scramble species nevertheless have strong preferences preferences that correspond to the male dimorphisms that have evolved in more derived species in their genus. The biases appear to be associated with recognizing signs of health prior to making choices about which fish to school with. The male dimorphisms that have evolved later appear to be supernormal stimuli that exploit these biases.
This is most interesting. I shall see if I can find out more about his work.
Edited by Dr Adequate, : No reason given.

  
Dr Adequate
Member (Idle past 284 days)
Posts: 16113
Joined: 07-20-2006


Message 29 of 36 (620382)
06-16-2011 1:32 AM
Reply to: Message 25 by Percy
06-15-2011 1:20 PM


Re: A Sexual Selection Question
The recent discussion prompts me to ask whether in species where males help raise the young, do we know whether drabness in males is more common? A drab male would be more likely to be around to help, so females might develop a greater preference (tolerance?) for drab males.
It would be hard for a female to know in advance whether a male would be a good provider. I guess in territorial birds she can look for a male with a large territory.
---
There are other ways for a bird to show off besides plumage. Singing, for example. I believe I've read of some cases where a male bird will actually sing itself to death in its exertions.
Or take bowerbirds. They are nothing special to look at, but their efforts in building bowers are remarkable. Here is the bower of a vogelkop bowerbird:
As you can see, the male is an extremely boring color --- indeed, so cryptic that you may have trouble finding it in the photograph. The bower and its decorations, on the other hand, are quite spectacular.
In this species the male plays no part in raising the chicks, nor is the bower itself of the slightest use.

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Replies to this message:
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Dr Adequate
Member (Idle past 284 days)
Posts: 16113
Joined: 07-20-2006


Message 32 of 36 (620402)
06-16-2011 7:45 AM
Reply to: Message 31 by aiki
06-16-2011 4:26 AM


Re: A Sexual Selection Question
In birds where the male is expected to feed the incubating female and later on the chicks, he can demonstrate his provider skills to the female with courtship feeding. He finds and offers food to the female, feeding her in the same way he would a begging chick. The extra food supplies also help bring her into breeding condition.
Interesting. Now besides being a display of competence, this is also an insurance of future behavior. It is true that courtship feeding doesn't in itself prove that he'll care for her and her chick after he's knocked her up, rather than running after another female --- but in order to successfully pursue another female, he'd have to expend the same courtship effort on female no. 2. So this female strategy tilts the cost-benefit calculation in favor of good providers and against philanderers.
And it's another ESS --- no individual can gain by bucking the system once it's in place.

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 Message 31 by aiki, posted 06-16-2011 4:26 AM aiki has replied

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Dr Adequate
Member (Idle past 284 days)
Posts: 16113
Joined: 07-20-2006


Message 36 of 36 (620969)
06-22-2011 3:56 AM
Reply to: Message 7 by slevesque
06-15-2011 1:07 AM


Drift: Beyond The ESS
I am positing that it does have a limiting affect because the preferences of the females is itself a selectable trait. Females who have preferences for characteristics who diminish fitness (bright colors) will have offsprings who will be at a disadvantage compared to offsprings of those who have preferences for less noticeable characteristics.
I've been thinking about this, and I did some computer simulations, and you turn out to be wrong.
You have in mind a situation where the female ideal of male beauty is itself a variable trait; for example the males vary in tail length (let's say) and the females vary in how long they like male tails (and perhaps also in how picky they are about this). This differs from my idea of how it usually works, in which all females would in principle prefer an infinitely long tail, but let's look at your hypothesis and see what would happen.
Now, consider that for any given female preferences, there is an optimum point at which males best satisfy the combined demands of natural and sexual selection; at any given time we expect the average value of the male trait to be close to this moving target because of the selective pressures (but not usually dead on it because of genetic drift).
Now, consider the case where the males tend to have the male trait to a degree that is (even slightly) below the optimum. Then females with a preference for greater values of the male trait have a selective advantage, because they will have a greater tendency to choose males closer to the optimum, which will therefore have a higher net fitness, which will be inherited by her children. Similarly, when males tend to have the male trait in excess of the optimum, females with a preference for smaller values of the male trait will be favored.
That is the selective pressure on female preference. It has nothing to do with what would be good for the species; it is driven by the drift of the values of the male traits around the optimum; that is, on a random factor.
(Of course, female preference is also subject to genetic drift, which is another random factor.)
Computer simulations bear this out. Selective pressures keep the male trait near the ever-shifting optimum; meanwhile female preference goes on what looks for all the world like a random walk; and if there is any bias in the randomness, it certainly does not seem to be in the direction of eliminating the male trait and female preference.
Edited by Dr Adequate, : No reason given.

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