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Author Topic:   The limitations of Sexual Selection
slevesque
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Posts: 1456
Joined: 05-14-2009


Message 1 of 36 (620216)
06-14-2011 2:23 PM


Ok, so I have been thinking about this for some and decided that it could make a worthwhile discussion here. Note that this has little to do with creationism, I just want to know if my understanding of sexual selection is accurate.

Now I don't deny that sexual selection is a real phenomenon, and in fact Endler's well known experiment on guppies is a classic example of it. My question is rather on how much explanatory power does sexual selection actually have ?

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 ?

A corrollary of this would be an extension of Endler's experiment: What if we had let the guppies with predation for an extended period of time, long enough for NS to fix dull color preference in the female population. When this was reached, if we then removed the predators from the environment, the females should continue to select with dull colored males.

Am I missing something ?


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Message 2 of 36 (620218)
06-14-2011 10:05 PM


Thread Copied from Proposed New Topics Forum
Thread copied here from the The limitations of Sexual Selection thread in the Proposed New Topics forum.
    
Omnivorous
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Posts: 3808
From: Adirondackia
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Message 3 of 36 (620223)
06-14-2011 10:50 PM
Reply to: Message 1 by slevesque
06-14-2011 2:23 PM


Slevesque writes:

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 ?

At first blush, several things occur to me.

First, you are assuming that predators rely on the same perceptual apparatus (color vision) used by the female guppies to detect fitness in the male. Many marine predators instead use other senses: echolocation (dolphins), for example, or a bio-electrical field disturbance mechanism (sharks). Indeed, the rainbow panoply of marine life around coral reefs suggests that bright colors are not an insurmountable hazard. Also of note, many terrestrial creatures warn of their toxic character with bright colors.

Second, you posit that natural selection must limit sexual selection, but then assert sexual selection has explanatory power "only in the complete absence of predators"--you have no grounds to suggest the limiting effect of natural selection is absolute.

Finally, if there are color-targeting predators present, perhaps the brightly colored males display their fitness by surviving despite their attention-grabbing garb. The logic of sexual selection dictates that the selected trait denotes fitness, and perhaps sufficient speed and wariness to escape predators in this context is that fitness.

It's an interesting dynamic, though, and a great topic for discussion.



Dost thou think, because thou art virtuous, there shall be no more cakes and ale?
-Shakespeare

Real things always push back.
-William James


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crashfrog
Inactive Member


Message 4 of 36 (620227)
06-14-2011 10:58 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.

A female who prefers the most brightly colored mate in the vicinity isn't going to be subject to adverse selection as a result. And she's always going to pick some mate, therefore her genes are always going to be passed on, assuming she lives long enough.

So where this preference is widespread, males will equilibriate at the greatest, boldest coloration they can maintain in the face of predation. Shouldn't they?


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AZPaul3
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Posts: 3945
From: Phoenix
Joined: 11-06-2006
Member Rating: 3.9


Message 5 of 36 (620228)
06-14-2011 11:03 PM
Reply to: Message 1 by slevesque
06-14-2011 2:23 PM


Hey slev,

The female preference for colorful mates is spurred by the fact that such displays are signs of health and strength. Obvious evolutionary advantages for her offspring.

From the male perspective, regardless of the predation dangers, the greater chance at greater reproduction potential is also an evolutionary advantage.

Females who prefer bright-colored males (as signs of health, etc) have stronger healthier offspring and that includes her daughters who inherit her mating preferences.


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Dr Adequate
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Posts: 16094
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(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.


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slevesque
Member (Idle past 2775 days)
Posts: 1456
Joined: 05-14-2009


Message 7 of 36 (620251)
06-15-2011 1:07 AM
Reply to: Message 3 by Omnivorous
06-14-2011 10:50 PM


First, you are assuming that predators rely on the same perceptual apparatus (color vision) used by the female guppies to detect fitness in the male. Many marine predators instead use other senses: echolocation (dolphins), for example, or a bio-electrical field disturbance mechanism (sharks). Indeed, the rainbow panoply of marine life around coral reefs suggests that bright colors are not an insurmountable hazard. Also of note, many terrestrial creatures warn of their toxic character with bright colors.

Well of course, this assumption isn't really an assumption, it is more a given that the examples that pertain to my questions are those in which this is the case; ie that the choices of a female have an effect on the fitness of the males within a given environment.

Obivously, the cases where sexual selection is made on things that do not affect fitness, for example the song of a male bird (I doubt a predator cares what song a bird makes before he eats it), are not of interest here.

Second, you posit that natural selection must limit sexual selection, but then assert sexual selection has explanatory power "only in the complete absence of predators"--you have no grounds to suggest the limiting effect of natural selection is absolute.

I am more asking what the limiting relationship of NS on SS is.

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.

Finally, if there are color-targeting predators present, perhaps the brightly colored males display their fitness by surviving despite their attention-grabbing garb. The logic of sexual selection dictates that the selected trait denotes fitness, and perhaps sufficient speed and wariness to escape predators in this context is that fitness.

Of course other traits affect fitness, but it doesn't affect what I am saying here.

Edited by slevesque, : No reason given.


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slevesque
Member (Idle past 2775 days)
Posts: 1456
Joined: 05-14-2009


Message 8 of 36 (620252)
06-15-2011 1:14 AM
Reply to: Message 4 by crashfrog
06-14-2011 10:58 PM


A female who prefers the most brightly colored mate in the vicinity isn't going to be subject to adverse selection as a result. And she's always going to pick some mate, therefore her genes are always going to be passed on, assuming she lives long enough.

She isn't going to be subject to adverse selection, but her offspring will, and by extension her own preference for colors will be selected against.

So where this preference is widespread, males will equilibriate at the greatest, boldest coloration they can maintain in the face of predation. Shouldn't they?

Well if the preference is widespread, in that the preference for bright colors is a fixed trait in the femal population, then I can see how an equilibrium will form.

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.


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slevesque
Member (Idle past 2775 days)
Posts: 1456
Joined: 05-14-2009


Message 9 of 36 (620253)
06-15-2011 1:15 AM
Reply to: Message 5 by AZPaul3
06-14-2011 11:03 PM


The female preference for colorful mates is spurred by the fact that such displays are signs of health and strength. Obvious evolutionary advantages for her offspring.

Any references for this ? I have a hard time thinking this is true.


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AZPaul3
Member
Posts: 3945
From: Phoenix
Joined: 11-06-2006
Member Rating: 3.9


Message 10 of 36 (620254)
06-15-2011 1:18 AM
Reply to: Message 7 by slevesque
06-15-2011 1:07 AM


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.

Bassakwards, slev. It appears the bright colors signal a stronger healthier male. A sickly, weak, disease-ridden male cannot afford to put energy into much of a mating display.

And don't be thinking that longevity is the determinant here. Only reproductive success is the determinant as far as evolution is concerned. You may be eaten early but if you leave behind a few dozen kids you are much more "fit" then your drab brother who lives long and old but can't get laid.


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crashfrog
Inactive Member


Message 11 of 36 (620256)
06-15-2011 1:21 AM
Reply to: Message 8 by slevesque
06-15-2011 1:14 AM


She isn't going to be subject to adverse selection, but her offspring will

I don't see how. Her daughters are going to be unharmed by inheriting the preference - or perhaps benefited by it, if the males they're led to mate with are the strongest and fastest. The males are going to benefit by inheriting traits that females prefer, in addition to the increased strength and speed their fathers must have had to survive predation without camouflage.

It's evolutionary benefits for all concerned, seems to me.

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.

Female mate preference for bold coloration is fitness-friendly for females; the only pressure against it is the pressure of increased susceptibility to predation in males due to easy-to-spot coloration. And that pressure applies only to males.


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AZPaul3
Member
Posts: 3945
From: Phoenix
Joined: 11-06-2006
Member Rating: 3.9


Message 12 of 36 (620257)
06-15-2011 1:24 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.

Wiki is the quickest easiest resource. The peacock? Birds of Paradise? Maybe should have read prior to the OP?

Also see the Good Gene Theory

Edited by AZPaul3, : No reason given.


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slevesque
Member (Idle past 2775 days)
Posts: 1456
Joined: 05-14-2009


Message 13 of 36 (620258)
06-15-2011 1:26 AM
Reply to: Message 6 by Dr Adequate
06-14-2011 11:12 PM


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.

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.

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.

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 ? In other words, we could then remove the predation pressure and the population would remain camouflage colored because of sexual selection.

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, 1992.)

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.)

Interesting post, and I'll have to reread it again tomorrow to comment on it as right now my brain is in shutdown mode.


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Dr Adequate
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Posts: 16094
Joined: 07-20-2006
Member Rating: 5.5


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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slevesque
Member (Idle past 2775 days)
Posts: 1456
Joined: 05-14-2009


Message 15 of 36 (620260)
06-15-2011 1:32 AM
Reply to: Message 12 by AZPaul3
06-15-2011 1:24 AM


I did, but you'll need to highlight where it is shown that males with brighter colors are healthier.

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.


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