Claudia

Removal of a stress is a change of environment. Evolution of jawbones is influence by removal of a stress: less need to strong jawbones because food is smoother, and because dentists are better, less risk of infection due to wishdom teeth which do not have enouh space. Added to the fact that wisdom teeth ae also disapearing, a significant minority of people miss some or all of them.

pz

'Use it or lose it' has Lamarckian implications, which is why I mentioned it.
I don't have much data on wisdom teeth, unfortunately, but other similar factors show interesting patterns. The Flynn effect and height, for instance -- there we see multi-generational trends. Populations are changing, the changes are persistent, and we are seeing patterns of change. It can't be explained genetically. It is all problematic and we can't explain it very well, and that's precisely what is so interesting about it.

pz

Why are they disappearing? The point here is not to deny the changes, but to mention that they are not easily accommodated within a purely Darwinian explanation. Examples of Darwinian explanations would be that a century ago, small-jawed individuals were dying of impacted wisdom teeth in significant numbers, or that modern individuals are preferentially reproducing with small-jawed mates. I don't see that either of those are particularly tenable.

I favor the idea that changes in diet lead directly to a plastic developmental response in jaw growth, as you suggest. I also think it is an example of evolution. I just want to be clear that this would definitely not be an example of Darwinian evolution.

Jay30

There was an article in the October issue of Scientific American about how skin colour of humans has evolved as a result of migration and is determined by being light enough to allow enough sunlight to produce vitamin D but dark enough to prevent UV rays destroying vitamin B folate, which can cause birth defects (ie spina bifida).

Notable exceptions are the Inuit who have been here (northern Canada) for 5,000 years but have darker skin than would be expected but have a diet consisting largely of sea food which is high in vitamin D; also, the Arabian people who migrated from Europe about 2,000 years ago don't have skin as dark as other Africans at the same latitude but adjusted mainly with clothing and shelter from the damaging effects of UV radiation.

This is in contrast to previous theories that darker skin was to prevent skin cancer because that generally occurs later in life -- past the reproductive years.

Walross

I had a thought about the problem of wisdom teeth. Of course I have no research to back me up, but maybe humans (on average) show preference for mates with finer features. I know this is currently true in our culture when it comes to males selecting females. If that were the case throughout our evolution, then smaller jawed females would be giving birth more often than their large jawed sisters. Of course, I have no idea why it wouldn't be the other way around - it seems to me that it would be more of an advantage to have a large jaw with more useful teeth. Like I said, it was just a thought,

Walross

scigirl

Hello Jay30, and welcome to infidels.

Feel free to introduce yourself <a href="http://iidb.org/cgi-bin/ultimatebb.cgi?ubb=forum&f=43" target="_blank">here</a> if you so desire.

Ok, back to your regularly scheduled discussion...

scigirl

Claudia

pz, may be the spreading of short jaw genes is due that there is less drive to chose mates on physical strength of the jaws, as it is less useful that sometimes earlier? Brutish look being less fashionable, because when the basic urgencies of life (to get food) are solved, people put more importance on secondary things?
We know that beauty criteria do change with the time. Why would it not be one of the reasons, because removal of a stressing environmental condition allows it? small jaws implying a more appealing smile?

pz

Yes, it could...except that the pacing is all out of whack. Going from a situation where dental care is an agonizing risk to be avoided at all costs to one where it is practically required for good health in a few generations makes it unlikely that that is the explanation here. It's happened too fast to be accounted for by allele replacement, unless there is a truly overwhelming force of selection at work. We may be selecting for delicate-boned beauty, but I don't think we're selecting with quite the ferocity needed to explain this phenomenon.

Feather

Maybe it's an exponential thing? It wasn't noticeable 3,000 years ago, but has passed a sort of "e-folding" marker.

cricket

I think it's considered alluring to be fine-boned, if female. Not because it's useful to be fine-boned, but because it's the opposite of the male trait. The males that are ultra-testosterone laden have jaws that are large. (Like Tom Platz; the HGH hyper-jaw look). Because of that contrast the preference has evolved for a woman's jaw to be daintier. And as that preference has evolved, smaller jaws for women have been selected for.