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Author Topic:   The limitations of Sexual Selection
Dr Adequate
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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
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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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PaulK
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Message 18 of 36 (620265)
06-15-2011 1:55 AM
Reply to: Message 1 by slevesque
06-14-2011 2:23 PM


Some points to consider:

1) It is not unusual for there to be a trade off in evolutionary terms. Speed versus endurance for example. Or the enormous energy cost of the human brain. So long as sexual selection confers a sufficiently strong advantage to offset the disadvantage of increased vulnerability it's a win.

2) In birds it is quite common for males to have colourful plumage while the females are much duller. The males are usually less vulnerable (since they do not sit on nests) - and in some species are relatively expendable (if they are not required to help raise the young).

3) We must also not forget that any variation in male appearance that was really problematic would likely be culled from the population before reaching breeding age, so sexual selection doesn't have that great a slope to be climbed.

4) The question would be the spread of the trait rather than the initial acquisition. A preference in itself does not have an evolutionary disadvantage. A drabber mate will do if that is all that is available. An initial variation in the male population is also to be expected. At this stage the trait could spread even by drift. There is no significant disadvantage to the female, and we may assume that the normal variation in male appearance is not greatly problematic either. If the female preference conveyed an evolutionary advantage as well (e.g. brighter appearance correlated with health) then it would have an even stronger tendency to spread.


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AZPaul3
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Message 19 of 36 (620267)
06-15-2011 2:23 AM
Reply to: Message 15 by slevesque
06-15-2011 1:32 AM


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.

quote:
Wiki on Sexual Selection

The peacock provides a particularly well known example of intersexual selection, where ornate males compete to be chosen by females. The result is a stunning feathered display, which is large and unwieldy enough to pose a significant survival disadvantage. Biologists have suggested that the layers of the ornate plumage of males provide a means of demonstrating body symmetry, such that peahens are "trying" to discover the health of the male or the quality of his genes. Diseases, injuries, and genetic disorders may impair the body's symmetry. There is also evidence that reduced symmetry of males may result in fewer offspring, or allow males access only to females with lesser parenting skills.[14]


As Dr. A has already stated, a dull plumage could be a sign of disease or genetic anomaly, neither of which the female would accept.

In a species where sexual selection is based upon display traits any health/strength/genetic issues may have significant effects on the display and that most certainly pertains to the vibrancy of the color.

In other species it may be the size of the horns or the finesse of the dance.


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Dr Adequate
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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
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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.


  
Omnivorous
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Message 22 of 36 (620283)
06-15-2011 6:57 AM
Reply to: Message 8 by slevesque
06-15-2011 1:14 AM


Slevesque writes:

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.

Either you have specific knowledge that guppy predators use color vision to detect them, or you assumed so. Hard cases make bad law; flawed examples yield flawed conclusions. You can, of course, abandon the guppies as an example.

In some species, sexual selection works congruently with natural selection: bigger, stronger males who thus win head-butting contests, for example, also more effectively repel predators. If we are to examine the phenomena of sexually selected traits that increase susceptibility to predation, you first need to find some. Without those, we are at sea in abstractions, and awash in assumptions.

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.

That's not what I'm saying. I am saying that sexually selected traits are intrinsically linked to reproductive fitness.

Even if we assume (as you have) that bright coloration increases predatory attention, we also know that sexually selected display traits denote reproductive fitness precisely because the displayer can afford them. Individuals who attempt displays they cannot afford are likely to be culled by predators or the consequences of metabolic extravagance--thus natural selection pressures "enforce" sexual selection. If there were no cost to the sexually selected display, there would be no benefit to sexual selection.



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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caffeine
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Message 23 of 36 (620285)
06-15-2011 7:24 AM
Reply to: Message 22 by Omnivorous
06-15-2011 6:57 AM


Why guppies?
Either you have specific knowledge that guppy predators use color vision to detect them, or you assumed so. Hard cases make bad law; flawed examples yield flawed conclusions. You can, of course, abandon the guppies as an example.

The guppies are a good example, because it's been demonstrated that increased predator pressure selects for less conspicuous guppies. Slevesque mentioned Joh Endler's experiments, which are summarised here. He stocked different experimental pools with different variations of guppies and different numbers of predators. In those with the most predators, the guppies with the drabbest colours were strongly selected for.

Edited by caffeine, : No reason given.


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Omnivorous
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Posts: 3808
From: Adirondackia
Joined: 07-21-2005


Message 24 of 36 (620287)
06-15-2011 7:37 AM
Reply to: Message 23 by caffeine
06-15-2011 7:24 AM


Re: Why guppies?
Thanks, caffeine. That's what I wanted to hear from Slevesque.

I know I could have searched for it myself, but it was his argument's burden.


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Percy
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Message 25 of 36 (620318)
06-15-2011 1:20 PM


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.

--Percy


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PaulK
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Message 26 of 36 (620322)
06-15-2011 1:35 PM
Reply to: Message 25 by Percy
06-15-2011 1:20 PM


Re: A Sexual Selection Question
The only example I know of is the mandarin duck, although there could be others. Even then the male stays away until the ducklings have hatched. Even that is unusual in a duck, and they are among the most obvious groups with showy males and drab females.
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ZenMonkey
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Message 27 of 36 (620373)
06-15-2011 10:25 PM
Reply to: Message 25 by Percy
06-15-2011 1:20 PM


Re: A Sexual Selection Question
Percy writes:

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.

I believe that this is indeed the case. As I recall, it's also the case that when the male is more involved in the rearing of offspring, there is also less sexual dimorphism in general. The boys start looking more like the girls.

ABE: But I've been wrong before.

Edited by ZenMonkey, : No reason given.


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Boof
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Message 28 of 36 (620375)
06-15-2011 11:07 PM
Reply to: Message 27 by ZenMonkey
06-15-2011 10:25 PM


Re: A Sexual Selection Question
ZenMonkey writes:

Percy writes:

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.


I believe that this is indeed the case. As I recall, it's also the case that when the male is more involved in the rearing of offspring, there is also less sexual dimorphism in general. The boys start looking more like the girls.

The Emu is a good example of this I think. Wiki has quite a good article on them.


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Dr Adequate
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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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caffeine
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Message 30 of 36 (620388)
06-16-2011 3:54 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.

And if a male is helping to raise the young, it's less helpful for him to be bright and colourful as well. Ensuring the young he's investing all this time and energy in do well is more important than picking up another lady.

For other examples, see albatrosses, or Emperor penguins. both divide childcare equally, and males and females are pretty indistinguishable.

I'm trying to find a more exhaustive look at this than the arbitrary examples we're coming up with, but all I keep coming across on the interwebs are statements that it's (generally speaking) true.

ABE: So maybe the pattern isn't as tidy as I thought. I just found this article on sexual dimorphism on birds, which I don'd currently have the time to read in full. The abstract claims that, whilst size dimorphism follows the predicted pattern of correlating with differences in parental care, plumage-colour dimorphism doesn't. Essentially, it's correlated with whether or not males sleep around, which makes sense.

Edited by caffeine, : No reason given.

Edited by caffeine, : No reason given.


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