Freethought & Rationalism Archive

Nic Tamzek · 06-11-2003, 08:25 PM

[goes back, actually reads thread and discovers that the spandrel possibility was raised long before my entry...]

FWIW, I do think it is worthwhile to have (novel) speculative hypotheses, adaptive or otherwise, published in the peer-reviewed literature, at least in journals that have such things as part of their mission. It's quite worthwhile to have the hypothesis proposed clearly and to have the authors amass all the evidence that they can for or against, and to propose what tests they can think of. If readers uniformly find it unconvincing and dismiss the hypothesis, well then, that's progress also.

For complex reasons, evolutionary speculation has a worse reputation than, say, speculation about the detailed workings of biochemistry. I must have half a dozen pdfs that are reviews of the question "how the bacterial flagellar motor actually works" -- i.e., how proton flow is coupled to rotation -- proposing and debating a half-dozen different models. Somewhere between most and all of these models are wrong, but that's not a problem, sooner or later one of them will emerge victorious.

One might argue that evolutionary scenarios are harder to test, but I'm not sure this is true. Thousands of papers have been spent on the flagellum, and they still don't have the exact data they need to pick the right model. You almost never have the data that you want in science.

nic

PS: One last point. I think that "spandrels" arguments can be just as dubious as adaptationist arguments. Gould et al. accused adaptationists of "just-so stories", but what is the spandrel-type "the feature arose as an accidental by-product" but yet another just-so story (and rather more like Kipling's actual stories, at that)? The key, as always, is testability and weight-of- evidence. IMO if you have a potentially testable hypothesis then you're not engaging in irresponsible just-so storytelling.

Autonemesis · 06-11-2003, 08:27 PM

Quote:

Originally posted by KC
What concerns me is, if parasitic infection was the driving force behind hairlessness, then the fact we do NOT see more of it in our primate cousins makes me question what is so compelling about the hypothesis in the first place. Bringing in additional factors which favor hairlessness, like man's use of clothing, only weakens the justification for the original hypothesis, in my opinion.

It seems just as plausible to me to reason that hairlessness is culturally evolved and encouraged by the religious notion that man is superior to the beasts, which embeds a subconscious connection with hairlessness, cleanliness and godliness.
As evidence of this bias in the human cultural subconsciousness, as it were, I offer this quote:

Quote:

"The question we always have in explaining unique human traits is: why didn't other animals evolve them as well if they are so advantageous?" he (Pagel) told New Scientist.

Who can say that these "unique human traits" had or will have any advantage to the long term survival of homo sapiens? We've only been around a few million years. That's a lot less time than, well, almost every other species that endures today. Seems a little premature to endow these traits with super-survival powers just yet. If you ask me, the key trait that endows a species with long term endurance is ultra-simplicity. The "more evolved" your species is, the shorter time it seems to endure on the planet. Bacteria, algae, sponges, and other simple forms are the survival champs, not humans.

Can I publish now?

Nic Tamzek · 06-11-2003, 08:34 PM

ps418 wrote provocatively,

Quote:

I'm interested in learning about how adaptationist hypotheses are properly tested. Could you list some examples of properly tested adaptationist hypotheses? Are there any features of H. sapiens that you'd regard as adaptations? If so, what are they, and what evidence suggests that they indeed adaptations rather than spandrels? Please don't mistake this for a rhetorical question, because I'm genuinely curious.

I'll take a shot. Off the top of my head:

The high frequency of adult lactose metabolism capability in herder-derived cultures
Sickle-cell anemia heterozygosity for malarial resistance
Alcohol dehydrogenase (memory is fuzzy here)

In a less directly testable vein, but still well-supported IMO:
The design of human hands for tool use
The fairly radical modification of human feet for walking, from their hand-like ancestors

Things that are fairly obviously "for" something are fairly common in biology. Gould et al.'s point was never to deny this, I think, they just pointed out that not every novelty in evolution had to be a result of selection "for" something.

Nic Tamzek · 06-11-2003, 08:40 PM

While I'm at it: in consideration of the ectoparasite hypothesis...it seems to me that the protection *from* bugs offered by hair is at least as important as the habitat *for* bugs offered by hair.

Particularly in Africa, mosquitos and biting flies (tsetse flies, particularly) are very non-trivial hazards. Certainly dense enough fur would offer some protection. I suspect that a chimplike ancestor's hair wouldn't be dense enough in the first place.

Like I said before, MHO is that human hairlessness is An Official Mystery. We need to know what the genetic mutations were that caused in in the first place, for starters. It could, after all, have been a fair simple and late-arising mutation, for all we know. Humans have bred hairless dogs without much trouble:

http://www.nhm.org/exhibitions/dogs/...rtificial.html

Nic Tamzek · 06-12-2003, 03:42 AM

Hey, here is a very intelligent essay on thermoregulation and body hair.

Human Thermoregulation and Hair Loss
http://www.modernhumanorigins.com/anth501.html

An additional important factor we haven't considered yet is sweating, which becomes and effective heat-removal mechanism as hair is reduced.

Also, body size:

Quote:

Although hair can impede heat loss through sweating, it is important in thermoregulation for maintaining body heat in either colder environments or at night and for the reflectance of solar radiation away from the body. Dense hair cover is very efficient for both of these purposes, and it is an important question to ask why the selection for increased sweating efficiency for heat loss would be more important than the selection for the heat retention mechanism of hair cover and the heat prevention mechanism of solar radiation reflectance. Dense hair cover is an effective heat retention device, but only in smaller animals. As animals in increase in size, the effectiveness of hair cover decreases. This is due to the reduced ratio of skin area to volume as mass increases. As body size increases, the amount of metabolically derived heat increases dramatically, but the ability of the organism to effectively lose this heat is retarded by the decreased ratio of surface volume. Thus, the percentage of heat lost to the environment by conduction decreases, simply because the organism loses the ability to lose heat as size increases. This means that the hair cover is less and less evolutionary meaningful for the retention of body heat. The corollary of this axiom is that as body size increases, and the metabolic heat load increases, there is an increased need for mechanisms to remove heat in hotter environments or in periods of high metabolic heat production. So as body size increases, dense hair becomes less and less effective at retaining body heat, and more and more maladaptive for removing body heat (Schwartz & Rosenblum 1981; Robertshaw 1985).

The obvious solution to this situation is decreased body hair with increasing body size, which is exactly what is seen in anthropoids. When the number of hair follicles present in species per unit of area is compared with body size, all primates (including humans) fit along a regular log linear regression line, along which the density of hair per unit of area decreases as body size increases. Species like chimpanzees and gorillas have relatively fewer hair follicles per unit area of skin compared to the smaller monkeys. Humans fall along this line, and have a relative hair density almost the same as seen in chimpanzees, gorillas and orangutans. The difference between the thick pelage of the Great Apes and humans is not in terms of the density of hair, but in its length and thickness and the production of vellus hair in most humans to the exclusion of terminal hair on the body. Humans are not "hairless", but are merely covered by thinner, smaller and unpigmented hair (Schwartz & Rosenblum 1981; Schultz 1931).

KC · 06-12-2003, 06:00 AM

Another thing we might consider is the possibility that several factors acting together might be responsible for the hairlessness of man. For example, imagine thermoregulation providing the impetus for hairlessness originally, and ectoparasite relief reinforcing the trend once it started.

KC

KC · 06-12-2003, 06:13 AM

Nic writes:

Quote:

Like I said before, MHO is that human hairlessness is An Official Mystery. We need to know what the genetic mutations were that caused in in the first place, for starters. It could, after all, have been a fair simple and late-arising mutation, for all we know. Humans have bred hairless dogs without much trouble

I'm not sure special mutations are particularly necessary, other than just strong directional selection on existing variation of 'hirsuteness'. That's what makes me wonder about its absence in other primates.

KC

ps418 · 06-12-2003, 06:43 AM

Nic,
Good points all around. This gives me plenty to think about.

Quote:

Nic's quote:
As someone just mentioned, it's not even clear that the loss of hair in humans is even adapted/"designed" for anything in particular. Could be sexual selection (it is worth comparing male beards to the manes of other primates)

As I pointed out in my last post, it is possible that reduced hair cover is a sexually selected trait that is also naturally selected, or was initially naturally selected and subsequently exaggerated by sexual selection. The significant sexual dimorphism for hair cover is at least suggestive that the trait has been under sexual selection.

Quote:

Sir Ronald Fisher. . . emphasized that sexual selection typically relies upon a trait having a naturally selected advantage to begin the process of its exaggeration (Fisher 1930). The ectoparasite hypothesis provides this advantage: initial naturally selected evolution towards reduced amounts of body hair may then have been reinforced by Fisherian or other forms of sexual selection as hairlessness-- by virtue of advertising reduced ectoparasite load-- became a desirable trait in a mate. Unusually among sexually selected traits, reduced body hair would be desirable in both sexes. Greater loss of body hair in females [would then plausibly follow] from the conventially stronger sexual selection from male versus female mate choice in humans.

Quote:

Nic:
While I'm at it: in consideration of the ectoparasite hypothesis...it seems to me that the protection *from* bugs offered by hair is at least as important as the habitat *for* bugs offered by hair.

Particularly in Africa, mosquitos and biting flies (tsetse flies, particularly) are very non-trivial hazards. Certainly dense enough fur would offer some protection.

I've been wondering about precisely this point. You suggest that the protection offered BY hair would provide a fitness advantage that is equal to our outweighs the protection of hairlessness. You may well be right. That certainly seems plausible, seeing as how my legs tend to accumulate fewer bites in the great outdoors than those of my wife and daughter.

On the other hand, I would strongly suspect that many animals WITH fur can still be bitten quite frequently, i.e. that biting flies have developed features that allow them to overcome the fur barrier, particulary the thin layers of fur that are(?) most common. Since the vast majority of potential tsetse food sources would be covered with fur, I'd expect adaptations to penetrate that fur would be common. Certainly tsetse flies can still cause major problems for furry animals. In other words, perhaps hair on most animals is only weak protection against biting flies, especially those with exposed areas, while hairlessness is very strong protection against most (relatively more stationary) ectoparasites. I will look for information on tstetse flies. But admittedly I'm speculating here, and if you can point me to some relevant literature here, then please do.

Quote:

Nic:
Like I said before, MHO is that human hairlessness is An Official Mystery. We need to know what the genetic mutations were that caused in in the first place, for starters.

Why do we need to pinpoint the mutations underlying our relative hairlessness before we can make inference about its benefits or lack thereof, or its status as either a byproduct or an adaptation? Would this apply to the morphology or human hands and feet as well? Obviously it would be useful to know what genetic mutations underly the between-species differences in hairiness, but I don't see that this is in any way a prerequisite for answering questions about fitness costs or fitness advantages of the phenotype itself.

Quote:

Nic:
It could, after all, have been a fair simple and late-arising mutation, for all we know. Humans have bred hairless dogs without much trouble:

That's true. I suspect that hair cover is controlled by several different loci, since there is a substantial range of individual differences in hair cover. But one locus (dominant, x-linked) significantly influencing hair cover on the face and torso has already been localized to xq24-27.1, though as I said this is probably only one of many loci influencing variation in hairiness.

Figuera, L.E., Pandolfo, M., Dunne, P.W., Cantu, J.M., and Patel, P.I. Mapping of the congenital generalized hypertrichosis locus to chromosome Xq24-27.1. Nature Genetics, 10:202-207, 1995.

See also:

Congenital Hypertrichosis Lanuginosa

Quote:

Nic:
Things that are fairly obviously "for" something are fairly common in biology. Gould et al.'s point was never to deny this, I think, they just pointed out that not every novelty in evolution had to be a result of selection "for" something.

If that's the case, then I'm not sure what all the hubbub and vitriol was about, since I've never seen anyone suggest otherwise, i.e. that all evolutionary novelties are the result of selection. There may be pandaptationists out there, but I've never spotted one in the wild, so they must be quite rare. Certainly there are instances where adaptationist hypotheses have been presented with a certainty not proportional to the supporing evidence, but that is not a problem with adaptationist thinking itself. And in any event, entertaining an adaptationist explanation for any and every feature is perfectly reasonable and even desirable, so long as you are comparing it to a byproduct or other type of nonadaptationist hypothesis at the same time, and are cognizant of the limitations of the evidence in any particular case.

Quote:

Nic:
FWIW, I do think it is worthwhile to have (novel) speculative hypotheses, adaptive or otherwise, published in the peer-reviewed literature, at least in journals that have such things as part of their mission. It's quite worthwhile to have the hypothesis proposed clearly and to have the authors amass all the evidence that they can for or against, and to propose what tests they can think of. If readers uniformly find it unconvincing and dismiss the hypothesis, well then, that's progress also.

As you might guess, I agree completely. I'd go farther and say that scientific progress would come to a screetching halt if all papers were lists of facts without any 'speculative hypotheses.' It just strikes me as brain-dead obvious that the desire to disprove other people's speculations is a major impetus to scientific progress.

Patrick

Oolon Colluphid · 06-12-2003, 07:08 AM

Quote:

Originally posted by ps418
On the other hand, I would strongly suspect that many animals WITH fur can still be bitten quite frequently, i.e. that biting flies have developed features that allow them to overcome the fur barrier, particulary the thin layers of fur that are(?) most common. Since the vast majority of potential tsetse food sources would be covered with fur, I'd expect adaptations to penetrate that fur would be common. Certainly tsetse flies can still cause major problems for furry animals. In other words, perhaps hair on most animals is only weak protection against biting flies, especially those with exposed areas, while hairlessness is very strong protection against most (relatively more stationary) ectoparasites. I will look for information on tstetse flies. But admittedly I'm speculating here, and if you can point me to some relevant literature here, then please do.

I was stumped by the biting flies business for a while too, till your post above. I think you are probably correct: being furry or furless probably makes little difference to susceptibility to airborne attack. Mosquitoes for instance just as readily attack other creatures as they do us; I remember reading in Spielman and D�Antonio�s Mosquito that great clouds of mozzies have been known to actually exsanguinate reindeer / caribou. And reindeer have pretty thick pelts. It may even be that hair offers the mosquito some protection: nestled down between the hairs, she would be sheltered from wind while she takes her blood meal, and might even be able to stay in place even if the host licks or bites at the area.

So hairlessness might make no difference to some parasites, yet be a significant advantage against the critters that cling on: reducing what they have to cling on to, giving them fewer places to hide, and also making it easier to remove them.

Cheers, Oolon

Amen-Moses · 06-12-2003, 07:15 AM

Quote:

Originally posted by KC
But it ignores the obvious fact that many other primates are hairy and have parasites. I think it reasonable to say that, if there was such strong selection pressure for hairlessness to avoid parasitic infestation, then man would probably not be the exception here.

Other primates get around the parasite problem by being nomadic as do many other mammals, burrowing creatures such as mole rats are hairless and they are also social nesters yet the common mole is not social and is hairy.

Hairlessness could be ONE of many ways of dealing with parasites which would only be selected for if the other ways are selected against, i.e when humans started moving away from a purely nomadic way of life.

Personally I think there are many different reasons combined rather than one single reason for any trait in any species.

Amen-Moses

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06-11-2003, 08:25 PM	#31
Nic Tamzek Senior Member Join Date: Jan 2002 Location: California Posts: 646	[goes back, actually reads thread and discovers that the spandrel possibility was raised long before my entry...] FWIW, I do think it is worthwhile to have (novel) speculative hypotheses, adaptive or otherwise, published in the peer-reviewed literature, at least in journals that have such things as part of their mission. It's quite worthwhile to have the hypothesis proposed clearly and to have the authors amass all the evidence that they can for or against, and to propose what tests they can think of. If readers uniformly find it unconvincing and dismiss the hypothesis, well then, that's progress also. For complex reasons, evolutionary speculation has a worse reputation than, say, speculation about the detailed workings of biochemistry. I must have half a dozen pdfs that are reviews of the question "how the bacterial flagellar motor actually works" -- i.e., how proton flow is coupled to rotation -- proposing and debating a half-dozen different models. Somewhere between most and all of these models are wrong, but that's not a problem, sooner or later one of them will emerge victorious. One might argue that evolutionary scenarios are harder to test, but I'm not sure this is true. Thousands of papers have been spent on the flagellum, and they still don't have the exact data they need to pick the right model. You almost never have the data that you want in science. nic PS: One last point. I think that "spandrels" arguments can be just as dubious as adaptationist arguments. Gould et al. accused adaptationists of "just-so stories", but what is the spandrel-type "the feature arose as an accidental by-product" but yet another just-so story (and rather more like Kipling's actual stories, at that)? The key, as always, is testability and weight-of- evidence. IMO if you have a potentially testable hypothesis then you're not engaging in irresponsible just-so storytelling.

06-11-2003, 08:40 PM	#34
Nic Tamzek Senior Member Join Date: Jan 2002 Location: California Posts: 646	While I'm at it: in consideration of the ectoparasite hypothesis...it seems to me that the protection from bugs offered by hair is at least as important as the habitat for bugs offered by hair. Particularly in Africa, mosquitos and biting flies (tsetse flies, particularly) are very non-trivial hazards. Certainly dense enough fur would offer some protection. I suspect that a chimplike ancestor's hair wouldn't be dense enough in the first place. Like I said before, MHO is that human hairlessness is An Official Mystery. We need to know what the genetic mutations were that caused in in the first place, for starters. It could, after all, have been a fair simple and late-arising mutation, for all we know. Humans have bred hairless dogs without much trouble: http://www.nhm.org/exhibitions/dogs/...rtificial.html

06-12-2003, 06:00 AM	#36
KC Senior Member Join Date: Mar 2002 Location: San Narcisco, RRR Posts: 527	Another thing we might consider is the possibility that several factors acting together might be responsible for the hairlessness of man. For example, imagine thermoregulation providing the impetus for hairlessness originally, and ectoparasite relief reinforcing the trend once it started. KC

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