Why do cave creatures lose skin pigmentation?

Sunlight activates the melanin in your skin, which gives it the color. So without the sun you end up being white. Caves have no sunlight.

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@BarbieQPickle,

What colour were the cavemen? Laughing

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@BarbieQPickle,

An important function of melanin is that it helps to regulate body heat--it gives off heat. So it is a benefit to people with brown skins living in hot sunny climates, and a liability to people living in cold climates.

However, i did not bring this up myself because i have no idea if this is true of the skin pigmentation of other animals.

Your statement is rather oversimplistic. Brown skinned people not only have more melanin in their skin, they have more melanin bodies which produce the pigment. It is the melanin bodies which give off the heat, and help them to keep down their body temperature. It should be obvious why melanin bodies and melanin would be liabilities to humans in cold climates.

I know of no positively, indisputably troglogdytic archaeological finds. I doubt that humans ever actually lived in caves--caves tend to be damp and poorly ventilated. It is far more likely that they used caves for various purposes, and lived in shelters in near-by overhanging bluffs facing south.

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@Setanta,

I had a biologist do a search this AM and she came up with something that suggests "multifctorial" effects of phenotypic expressions in Trogs. http://jhered.oxfordjournals.org/cgi/content/full/93/1/19

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@farmerman,

farmerman wrote:

I had a biologist do a search this AM and she came up with something that suggests "multifctorial" effects of phenotypic expressions in Trogs. http://jhered.oxfordjournals.org/cgi/content/full/93/1/19

Good article. Thanks FM.

Here are the interesting parts of the article (to me):

Quote:

In fact, a general evolutionary basis for character regression remains unknown to this day.

I suspected this to be the case, and that's what led to my question.

Quote:

There are two principal competing hypotheses to account for regressive evolution.
1. The first is that selection favors eye loss, perhaps for reasons of organismal or neural processing economy.

This is the one I anticipated for eyes, but which I couldn't use to explain pigment changes.

Quote:

2. An alternative is that the genes controlling the development of eyes become effectively neutral with the relaxation of selective constraints and are free to accumulate mutations impairing their function.

This is the effect that Brandon (and I) conjectured about earlier, but have no evidence for. I'm still not sure this isn't at least "part" of the de-evolution mechanism, but maybe not a big part.

Quote:

One previously unexplored approach to the problems of regressive evolution and troglobitic evolution is genetic linkage mapping... The study was small in scale, but the results provide the first direct evidence that troglomorphic traits are multifactorial... Two closely linked pairs of QTL were found. Each consisted of a regressive and a constructive trait QTL. These close linkages are unlikely to have occurred by chance and suggest that troglomorphic evolution might be facilitated by pleiotropy or by genetic hitchhiking.

Pleiotropy and Genetic Hitchhiking are the same mechanisms uncovered in the Silver Fox studies, except in this case the "linked" traits appear to be what we call "regressive".

So here's my follow-up question: If the loss of pigmentation and eyes are simply the result of random linkages to some other trait which is being selected for... then what is that other trait (or traits) which are being selected for? Whatever it is, seems to be in common for Crickets, Shrimp, Fish, Salamanders, and a host of other organisms. Since those organisms represent a couple of different phyla and several classes, the common factor must be pretty deep.

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@farmerman,

I hope everyone is enjoying this thread.

I never really understood the idea of Gene Linkage and Genetic Hitchhiking before. Especially surprising because I had brushed right up against the information back in 2004 and actually started a thread about it.

In the interim I encountered the story of the Silver Fox studies and grasped the meaning behind it, but never tied it back to the question of regressive evolution.

Now that I've been exposed to the same information again, I think I finally get it. Smile

I love it when I learn stuff on A2K.

1 Reply

@rosborne979,

sometimes more than several genes are involved in a single expression, with several others hanging around just to exert the "on/off" switches. Also sometimes one gene can have several expressions.

But in all cases, the genes are not the causes, they are the compatable results . Did I mention about the "fossil genes" and how they remain in the genomes of several animals including us? .

Im starting to fall way behinmd in my simplistic understandings of genic structure . Theres a whole series of upper levels of complexity recently written about . Because of the way the strands of DNA are smooshed within the cells nucleus, Ive read that sevearl animals have strands that, when extended, are several tens of yards long. Nowonder mutations occur at such high rates.

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It seems like there are several possibilities for explaining the loss of organs in cave creatures.

1. Genetic Hitchhiking (supported by literature provided by FM)
2. Possible Gene Degradation from lack of selective reinforcement (suggested by Brandon and myself)
3. Possible Energy Cost to the organism for unused morphology (suggested by Patiodog)

Is there any evidence to support conjectures #2 and #3?

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@rosborne979,

I think if you look at the prevalence of loss-of-functional-elements among parasites relative to their free-living cousins, there's a lot of common sense support for 2, and maybe for 3. Certainly neither notion originates on this thread.

And, per farmer's link:

Quote:

While hitchhiking and pleiotropy may play roles in regressive evolution, neither mechanism excludes an important role for drift of neutral mutations under conditions of relaxed selection, as outlined in the introduction.

1 Reply

@patiodog,

patiodog wrote:

I think if you look at the prevalence of loss-of-functional-elements among parasites relative to their free-living cousins, there's a lot of common sense support for 2, and maybe for 3. Certainly neither notion originates on this thread.

And, per farmer's link:

Quote:
While hitchhiking and pleiotropy may play roles in regressive evolution, neither mechanism excludes an important role for drift of neutral mutations under conditions of relaxed selection, as outlined in the introduction.

Agreed. All three factors may be in play, and the ideas didn't originate here.

I wonder what the relative potency of each factor has on evolution. I would have thought #2 and #3 would be pretty strong forces over time, but maybe they are completely overshadowed by #1.

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@rosborne979,

rosborne979 wrote:

It seems like there are several possibilities for explaining the loss of organs in cave creatures.

1. Genetic Hitchhiking (supported by literature provided by FM)
2. Possible Gene Degradation from lack of selective reinforcement (suggested by Brandon and myself)
3. Possible Energy Cost to the organism for unused morphology (suggested by Patiodog)

Is there any evidence to support conjectures #2 and #3?

I found this article which seems to support conjecture #3 as a contributing factor. The article talks about snails in particular, not cave creatures, however it does mention a hypothesis called "Energy Definition of Fitness" which seems very similar to what I was calling "Energy Cost".

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Why do cave creatures lose skin pigmentation?

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