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Posted: 05 February 2008 06:57 AM   [ Ignore ]   [ # 16 ]
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dougsmith - 05 February 2008 05:16 AM

I should add that it does make the point about some genetic changes in the last thousand years being due to disease pressure: to that extent, yes, there will always be some pressure for genetic change on a species-wide basis. Those who survive a disease tend to have some genetic difference that allows them to do so. And to the extent that increased population leads to increased disease pressure, there is at least a small amount of “speeding up” in that sort of genetic change. But we aren’t talking, e.g., about changes in morphology. We’re talking about minor changes at the cellular level that allow us to fight particular sorts of parasites, infections or viruses. And to the extent that now disease tends to be fought by scientific means, this sort of evolutionary pressure may lessen as well.

Perhaps I am still not understanding how evolution actually works in practice. But I imagine (perhaps incorrectly) that diseases would play a bigger rôle that what you seem to suggest. If group x is immune to a certain disease and at the same time the same group x carries an y genetic mutation (or several different ones) an epidemic such as the plague would have an enormous impact on helping to spread the y genetic mutation. The more people, the more epidemics, the faster the duplication of certain mutations.

[ Edited: 05 February 2008 07:03 AM by George ]
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Posted: 05 February 2008 07:27 AM   [ Ignore ]   [ # 17 ]
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George - 05 February 2008 06:57 AM

Perhaps I am still not understanding how evolution actually works in practice. But I imagine (perhaps incorrectly) that diseases would play a bigger rôle that what you seem to suggest. If group x is immune to a certain disease and at the same time the same group x carries an y genetic mutation (or several different ones) an epidemic such as the plague would have an enormous impact on helping to spread the y genetic mutation. The more people, the more epidemics, the faster the duplication of certain mutations.

Well in one sense we’re agreeing. Take the Black Plague. Arguably, most of the people who survived the plague had some slight genetic difference X that made them less sensitive to the bacterium. OK, so in that sense the plague would have made a change in gene frequency of the background population: after the plague, a higher percentage would have had X. But, firstly, this is going to be a minor change at the cellular level. We’re probably talking about changes in the immune system. And yes, our immune systems will change depending on the threats they face. Secondly, once the plague bacterium is no longer present in the population, genetic drift will tend to eliminate any changes that were made to the gene pool specifically to fight that disease. That is to say, the frequency of X in the population will tend to drift back to where it was before the plague.

We can tell from the fossil record that species are remarkably stable, even over tens or hundreds of millions of years. Once they find a general body plan that works in their niche, if they are have a sufficiently large interbreeding population, they remain morphologically unchanged.

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Posted: 05 February 2008 07:59 AM   [ Ignore ]   [ # 18 ]
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dougsmith - 05 February 2008 07:27 AM

Once they find a general body plan that works in their niche, if they are have a sufficiently large interbreeding population, they remain morphologically unchanged.

Well, maybe we have yet to find that plan for our bodies. In the Sprinting down the evolutionary highway article an anthropologist John Hawks says that, “We are more different genetically from people living 5,000 years ago than they were different from Neanderthals.”

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Posted: 05 February 2008 09:05 AM   [ Ignore ]   [ # 19 ]
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Gosh George.  I thought that Bigfoot lived in the Pacific Northwest of the United States and in Canada.  And the Yeti was to be found in Asia.  What makes you think that an unidentified hominid would be found in Africa?  LOL

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Posted: 05 February 2008 09:50 AM   [ Ignore ]   [ # 20 ]
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George - 05 February 2008 07:59 AM

“We are more different genetically from people living 5,000 years ago than they were different from Neanderthals.”

That sounds to me a hugely implausible claim. Based on what evidence?

Let’s remember that if we go back 5,000 years ago, we’re already in predynastic Egypt.

—Lyres, clarinets, board games, pottery, etc.

[ Edited: 05 February 2008 09:53 AM by dougsmith ]
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Posted: 05 February 2008 10:06 AM   [ Ignore ]   [ # 21 ]
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dougsmith - 05 February 2008 09:50 AM

Let’s remember that if we go back 5,000 years ago, we’re already in predynastic Egypt.

—Lyres, clarinets, board games, pottery, etc.

Well, the chimps are genetically closer to us than they are to gorillas, but their lifestyle resembles more closely the gorillas’ lifestyle than ours.

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Posted: 05 February 2008 10:31 AM   [ Ignore ]   [ # 22 ]
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Wouldn’t a large population be more conducive to genetic stability than genetic drift? Wouldn’t any mutation have to offer a very strong selective advantage to keep from being drowned in the larger population?

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Posted: 05 February 2008 10:48 AM   [ Ignore ]   [ # 23 ]
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Well, as for population size, it’s only 1 variable. Having passed through a genetic bottleneck, the human population has relatively low genetic diversity despite being large in numbers. This might slow diversification and selection to some extent because there isn’t much variation upon which selection can operate. If there were a large population with greater diversity, then isolation of subgroups might result in greater drift. But that raises another issue, which is that there is much more interbreeding between previously geographically-distinct populations than ever before. One of the reaons ther is some urgency to the Genographic Project I mentioned before is that it’s getting harder to find people who have been confined, in terms of genes, to ancestral locations for long periods on which to base a comparative genetic survey. So I think we shouldn’t get too hung up on population size alone as a variable.

As for genetic differences, I guess I’m skeptical of how we measure them. Clearly number of base pair differences and polymorphisms have some utility in looking at relatedness, but phenotypically they’re not that reliable. The oft-quoted fact that we differ from chimps by <1% of our genome (Human Evolution WIkipedia) iks misleading, since phenotypically there are significant differences that make the relative base pair similarity not so meaningful. SO I would say how are we measuring the genetic difference between present and ancestral human populations, and what exactly are we saying is its significance?

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Posted: 05 February 2008 11:17 AM   [ Ignore ]   [ # 24 ]
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Astyanax - 05 February 2008 10:31 AM

Wouldn’t a large population be more conducive to genetic stability than genetic drift? Wouldn’t any mutation have to offer a very strong selective advantage to keep from being drowned in the larger population?

Right, that’s what I’ve been trying to say ...

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Posted: 05 February 2008 11:20 AM   [ Ignore ]   [ # 25 ]
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George - 05 February 2008 10:06 AM

Well, the chimps are genetically closer to us than they are to gorillas, but their lifestyle resembles more closely the gorillas’ lifestyle than ours.

How so? IIRC chimp and gorilla lifestyle and culture are really quite different.

There are no morphological or behavioral differences between modern humans and humans in predynastic Egypt. Indeed, predynastic Egyptian humans are modern humans.

There are large morphological (and I believe behavioral) differences between chimps and gorillas.

There are significant morphological and behavioral differences between neandertal and modern humans.

So count me skeptical about John Hawks’s claim.

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Posted: 05 February 2008 11:31 AM   [ Ignore ]   [ # 26 ]
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Astyanax - 05 February 2008 10:31 AM

Wouldn’t a large population be more conducive to genetic stability than genetic drift? Wouldn’t any mutation have to offer a very strong selective advantage to keep from being drowned in the larger population?

This is probably a wrong analogy but memes have a better chance of survival in a big city as opposed to a small village. I know memes are not genes, but they both need to become fashionable if they are to be copied. And since small groups tend to be more conservative than larger groups, it might be easier for a mutation to have better chance of becoming accepted and attractive in a larger group.

[ Edited: 05 February 2008 11:38 AM by George ]
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Posted: 05 February 2008 11:38 AM   [ Ignore ]   [ # 27 ]
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George - 05 February 2008 11:31 AM

... small groups tend to be more conservative than larger groups ...

In what way? Do you mean politically conservative? Culturally conservative? If so, it is irrelevant to the question of genetic change. I would resist the analogy between memes and genes here. There is no obvious correlation. Or if there is, one can see it just as well from the other side: it is hard to get a meme to impact a city, since there are so many other competing memes. The problem is that then we think of mass media and its ability to distribute memes to large populations very quickly, etc. There is no process analogous to mass media for genes.

One has to be very careful about transferring “meme” talk into “gene” talk, and vice versa. They are really only loosely analogous.

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Posted: 05 February 2008 12:59 PM   [ Ignore ]   [ # 28 ]
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dougsmith - 05 February 2008 11:38 AM

There is no process analogous to mass media for genes.

Okay, I’ll try this: when the Spaniards came to South America, the native women painted body hair on their legs to mimic the European women (kind of comical to think that 500 later it’s the other way around). The “more European” you appeared the better chance you had of surviving in a big city; you were basically a lucky mutant if you were born with natural hair on your legs. There was a genetic shift in body hair in South America because the population of Europe was becoming larger and had to look for new means to survive, expanding to the rest of the world. In return, 500 years later Latin music becomes very fashionable, and the lucky mutants who can dance salsa become the stereotype of a sexy man, etc. The larger the population, the bigger the chance for a variety of mutations to appear and to become adopted.

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Posted: 05 February 2008 01:41 PM   [ Ignore ]   [ # 29 ]
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George - 05 February 2008 12:59 PM

Okay, I’ll try this: when the Spaniards came to South America, the native women painted body hair on their legs to mimic the European women (kind of comical to think that 500 later it’s the other way around). The “more European” you appeared the better chance you had of surviving in a big city; you were basically a lucky mutant if you were born with natural hair on your legs. There was a genetic shift in body hair in South America because the population of Europe was becoming larger and had to look for new means to survive, expanding to the rest of the world. In return, 500 years later Latin music becomes very fashionable, and the lucky mutants who can dance salsa become the stereotype of a sexy man, etc. The larger the population, the bigger the chance for a variety of mutations to appear and to become adopted.

AFAIK the possession or lack thereof of body hair hasn’t changed much over the last few thousand years. Fads and fashions come and go, but really they are too fleeting to make much difference to the genetic stock. Yes, it’s true that if these fashions persist for long enough there can be some selection pressure that will select for certain traits and against others. But I don’t see any proof of it here.

And interbreeding of different “racial communities” (insofar as there are real biological entities, “races”, which is relatively dubious) has tended to mix the genetic pot; but if anything, this is likely to stabilize the gene pool.

For more on these general subjects, check out genetic drift articles HERE and HERE. E.g.:

In a large population [genetic drift] will not have much effect in each generation because the random nature of the process will tend to average out. But in a small population the effect could be rapid and significant. ... Drift is much more important in small populations. It is important to remember that most species consist of numerous smaller inbreeding populations called “demes”. It is these demes that evolve.

(Of course, these “demes” will have to be reproductively isolated in order to evolve).

Now, genetic drift is one thing, and fitness-enhancing genetic changes are another. Insofar as our environment is different from what it was ancestrally, there could be additional selection pressures that would bring about genetic changes in the future. Who knows? However, given the size and mobility of our population, significant random genetic drift is highly unlikely.

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Posted: 05 February 2008 02:12 PM   [ Ignore ]   [ # 30 ]
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Interesting discussion.  I’ll add one more observation.  In the past few hundred years the ability of humans to move to different populations has increased dramatically, therefore, any mutation that occurs in one area is very likely to be sprinkled among many different groups fairly quickly.  This means that only a very minor advantage can cause that gene to become a major part of the human diaspora.  This, to some extent, could explain the genetic uniformity among humans.

Since humans seem more likely to disperse than many other species, it might be interesting to see what kind of genetic diversity exists in, say, rats, who also seem to disperse widely and easily.

Occam

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