Doubting Didymus

I have to agree with geotheo about Dr S's interpretation of the sexual selection theory.

I think you have evolutionary gigantism backwards. Peacocks did not evolve giant tails just because females like them (stay with me here), but because the females subconciously use them as indicators of the health of a male mate.

So, while the tails are huge because of sexual selection, the thing being selected is not, actually the actual tail, but the health of the mate.

(edit: I note that you talked about this, but I want to make the point that health indication is what is actually being selected for)

The same phenomena, (according to the evolution series you refer to) is theorised to have played a part in human brain development, but it most certainly is not the head size that is being selecte, but the level of intelligence as signified by large heads.

You are making a mistake in assuming that our brains are secondary to our head size. If head size was the primary selection then evolution would quite certainly favour individuals who 'cheat', by filling the space with cheaper, low-maintenance material like fat or bone. There is nothing to suggest that having bigger heads means we would be forced to fill it with brain.

You suggestion that intelligence has no survival advantage is pure imagination. Intelligence is such a massive survival advatage that we have lost all our other ones, like big teeth and muscles (compared to primates, as you seem fond of doing). Intelligence is the only thing that keeps our species alive.

Sexual selection probably did in fact work on the size of the head (among other things), just as it worked on the size of the peacock tail. However, your idea that intelligence developed as a side affect of this is misguided. sexual selection was really working for intelligence all along, using head size as a signifier.

The sexual selection theory of intelligence seeks to explain why we have so much more intelligence than we seem to need. I will outline the general theories for human heads, and for peacock tails, to make the point.

In peacock tails, tail feathers evolved for a survival purpose: flight. Later, sexual selection began to favour larger tail feathers, as signifiers of an individuals health. This began a positive feedback cycle, as new generations of male peacocks would not only have genes for big tail feathers in males, but also genes for preferring big tails, which he would pass on to his female offspring.

In human brains, intelligence evolved to aid survival in many obvious ways, but inevitably females began to evolve a preference for males who displayed this survival advantage. (inevitable, because evolution favours females who choose the best genes for their children).

The same positive feedback cycle would arise as in peacocks, but not just for the size of the head, but for any displays of intelligence. The TV series you refer to suggested that sexual selection was historically favouring such displays of brain power as musicality and humour, as I recall. I think this is an obvious element of society today. I will hesitantly say that I think displays of brainpower such as intellectual or literary achievement, musical prowess and a sense of humor are very significant selection pressures.

So it is not accurate to say that we have brains because big heads are sexy, but sexual selection is probably involved.

[ September 03, 2002: Message edited by: Doubting Didymus ]</p>

Dr S

(DD)… but because the females subconciously use them as indicators of the health of a male mate.
(S) Exactly-because you need robust health to support the oversized appendage.

(DD) The same phenomena, (according to the evolution series you refer to) is theorised to have played a part in human brain development, but it most certainly is not the head size that is being selecte, but the level of intelligence as signified by large heads.
(S) Well then you weren't paying close enough attention. The head size brought with it other sexual attractions--like talking--but you had to begin with a big brain.

(D) You are making a mistake in assuming that our brains are secondary to our head size.
(S) The size of our brain is directly tied to the size of our skulls. Try squeezing your brain into an adult chimps noggin sometime.

(D) If head size was the primary selection then evolution would quite certainly favour individuals who 'cheat', by filling the space with cheaper, low-maintenance material like fat or bone. There is nothing to suggest that having bigger heads means we would be forced to fill it with brain.
(S) This is where we lucked out. Being primates to get a large head the brain had to increase in size. Bone was too heavy for the spine and we don't have fat deposits there.

(DD)You suggestion that intelligence has no survival advantage is pure imagination.
(S) Your suggestion that I said that is pure imagination. But survival advantage in adults is counter balanced by survival disadvantage in infants.

(DD) Intelligence is such a massive survival advatage that we have lost all our other ones, like big teeth and muscles (compared to primates, as you seem fond of doing).
(S) We lost the choppers and most of the muscle before we first grew the brain. I am comparing us to primates because we are a species of primate. Just as I compare the size of peacocks tails not to other peacocks but to birds in general.
(DD) Sexual selection probably did in fact work on the size of the head (among other things), just as it worked on the size of the peacock tail. However, your idea that intelligence developed as a side affect of this is misguided. sexual selection was really working for intelligence all along, using head size as a signifier.
(S) Why am I misguided? The oversized head would have been what indicated an ape that was healthy enough to support the extravagance.

(DD) inevitably females began to evolve a preference for males who displayed this survival advantage.
(S) As Darwin noted you need to survive in order to reproduce.

(DD) The same positive feedback cycle would arise as in peacocks, but not just for the size of the head, but for any displays of intelligence. The TV series you refer to suggested that sexual selection was historically favouring such displays of brain power as musicality and humour, as I recall.
(S) But before you got humor and music and love letters you already needed a brain that is considerably larger than a Bonobos. While these perks definitely were sexual enhancements in themselves you would have already had to select for the larger brain to begin with before they could come into evolutionary play. Yes, they definitely part of positive feedback cycle, of sex & survival- but a larger brain was a prerequisite before the cycle could start.

(DD) So it is not accurate to say that we have brains because big heads are sexy, but sexual selection is probably involved.
(S) I still must disagree. It's difficult not to consider intelligence such a wonder, after all it's what we have the most of and we think it's sexy. But that's just species vanity.

Doubting Didymus

Exactly. The BRAIN is the selection advantage, not the head.

My point is that we have a big head because we developed a big brain, not vice - versa.

You seem to be suggesting that brain matter is lighter than bone. Brains are always heavy (water, you know). Bone can be light, especially if there is an advantage for it to be so, witness bird bones.

Also, even if it did have to be brain, it could just as easily be unused brain-like tissue. If the head was the selection advantage and not the brain, then selection would favour those who took the easiest path to big headedness and filled the space with some light tissue that does not require advance electronic ballet to create. This has not happened.

You said: "You are making an assumption. That intelligence is a survival strategy in humans. It is not."

Well, this is just not correct. Intelligence is a huge survival strategy. Forgive me if I have interpreted this wrongly.

No, that just isn't true. We lost our physical survival advantages at the same time as, and in near direct proportion to, our increase in brain size. Why we lost the physical survival adaptations is well understood: we were relying more on our brains.

No, it would indicate intelligence, not health. Big, heavy heads can not indicate our state of health. Our head is the same size no matter how sick we get, which is not true of peacock tails, or female breasts. A malformed, small head would indicate poor health only if there were problems with the individuals development. This does not support your argument, however, as it applies equally to arms and legs, etc.


What is your point? Natural selection would favour females who favoured intelligent males. Therefore, females (or males, come to think of it) who preferred intelligence would be the ones who survive and reproduce.

You are making my point for me. Intelligence was a survival advantage to begin with, completely unrelated to sexual selection for heads.

Also, a point about expressing your intelligence. You do not need to play bach to attract a mate. Any display that shows you to have good genes for a mates offspring would suffice. Using intelligence to scare away a rival is a good example, and happens quite often in the great apes.


The point I am trying to make is that we have big heads because we use big brains, not the other way around. I don't see what vanity has to do with it.

[ September 03, 2002: Message edited by: Doubting Didymus ]</p>

echidna

Out of curiosity, so what ?

Our physical limitations seem less and less important each day. Society's complexity seems to provide ample opportunity for an enormous diversity of human beings.

Further, physical limitations can actually on occasion give rise to other positive characteristics. Is it a problem ?

Doubting Didymus

Even if Christophers concerns are founded, the important point to consider is that it would take hundreds of thousands of years to even notice a small difference. You may as well worry about the sun exploding.

[ September 03, 2002: Message edited by: Doubting Didymus ]</p>

beausoleil

The fallacy lies only in reasoning from the cause in the difference between humans and apes to the cause of differences among humans.

The fallacy was pointed out by critics of 'The Bell Curve' - differences in a measure within a population and differences in the same measure between the average in different populations cannot be attributed automatically to the same thing.

Clearly the difference in size, structure and performace between the average ape's brain and the average human's is genetic. This does not have anything to do with whether the difference in size, structure and performance between any two individual humans is genetic.

I'm not saying it isn't genetic - just that the point made about apes isn't strictly relevant. I have no idea where the nature vs nurture debate is stuck these days.

Christopher Lord

Hundreds of thousands? we've transformed wolves into pets in a few thousand simply by removing their need to fight to live. breeders may have also played a small role, but I doubt there were many directed breeding programs in the stone age.

Mostly, wolves domesticated so dramatically because we FED them in return for assisting the hunt.

Once the pressure which created powerful weapons (killer instinct, big claws, fangs, venom, big brains, and so on) disapears, what becomes of the killer who posses them?

Evolution can engage in an ever-spiraling arms race, why not arms reduction when the pressure is gone?

Doubting Didymus

Wrong.

Dogs have been separate from wolves for as much as 100 000 years, and changed very little until we started up our intensive breeding programs.

look <a href="http://www.abc.net.au/rn/science/ss/stories/s438412.htm" target="_blank">here</a> for the most accessible discussion of this topic I could find. It is a transcript from Australian radio national science program that I listened to.

It takes hundreds of thousands of years for evolution to produce minor results in most cases, and it would be even slower when there are no pressures, such as the case of human intelligence.

In breif:
Artificial selection produces results in a few thousand years.

Natural selection produces results in hundreds of thousands of years.

Genetic drift (evolution in the abscence of pressures) would take even longer than natural selection to produce noticable effects. You are looking at many hundreds of thousands to the order of millions of years before everyone is stupid.

Doubting Didymus

A qualification to my previous post: I don't actually think there are no pressures on humans today.

While it may be true that there are no or very few negative pressures, (stupid people do not die anymore) positive pressures still exist in the form of mate selection, which I have discussed in a different context earlier. We are still subconciously attracted to indicators of intelligence in the form of intellectual or literary achievement, musicality and a sense of humour, to name a few.

echidna

Then why do blondes still have more fun ?