Global Warming

[dreadgeek]

Originally Posted by IslandScout

Interesting.

I'm sorry, I'm not getting the connection (why it's "Given" that (despite or because of?), our small generic diversity we've had one close brush with extinction. Can you explain again, please?[/quote]

Certainly. It's not that we had a brush with extinction *because* of our lack of genetic diversity. The small amount of genetic diversity among our species is an *effect* of that brush with extinction not the cause. A few of different things point toward this being the case. The first--and the one which, more or less, gives birth to the other two is that both the mitochondrial DNA variation (I'll refer to this as mtDNA from here on out) and the Y-chromosome variation is less than what we would expect given our numbers. All seven billion of us carry one of seven mtDNA variations. The roughly 49% of humans who care a Y-chromosome are all in some 15 groups. Again, this is smaller than one would expect. For example, chimpanzees have much more genetic diversity and worldwide they don't make up even 1% of human numbers but any two given chimps that are not closely related will share far fewer genes than any two non-related humans will.

This kind of anomaly cries out for explanation. Since mutations in all species accumulate at fairly predictable rates once you can work backward and start to see where those genes started and how they radiated out amongst existing human populations. What that work has shown is that every human being alive is related to a fairly small population, of ~10000 breeding pairs, that lived in sub-Saharan Africa in the range of 75 to 100K years ago.

The easiest way to think about this is the relationship between you and distant cousins. Somewhere out there are people whom you are distantly related to that you have no idea that you share genes with them. If, however, we sequenced your DNA and their DNA and, assuming we were lucky, had access to all the intermediary DNA we could work backward to where your shared genes come from. In much the same way, although obviously over a much longer time period, with a far larger sample and with lots of gaps in that sample, we can work back where various genes found in populations had their origins.

I won't bore you with the mathematics of it but there are certain things that just follow from certain observations. For example, since mtDNA is passed down, pretty much unchanged, from the mother it means that there must have been a mitochondrial Eve (in other words some woman or women who carried the copies of the mtDNA that all humans have). Even though evolution tends to homogenize a population because it favors those genes that work better in their environment, there's a *lot* of genetic diversity that is just missing in our species and the only real explanation for it--given that we know the mutation rate in the species--is that a lot of the diversity was wiped out and then bounced back.

As far as how we might adapt as a species is anyone's guess. We are still evolving as a species but we've changed the rules of the game so much that it's just this side of impossible for us to know what kinds of traits are being selected for.

I wonder if we're selecting for traits enabling people to have more resistant to environmental toxins.

We may be. It's hard to know. There's different levels of selection as well. For instance, purely by happenstance we likely *are* being selected for disease resistance because living in cities exposes you to far more pathogens than living in a low-density hunter-gathering-fishing village. The problem is that sometimes, we select for things that have no adaptive function *other* than that in a certain population certain traits may become 'fashionable'. For instance, if red-hair is considered to a mark of beauty within a population then, on average, more red-haired children will be born. Several hundred generations down the road, there's a a lot of red-haired people around. No one remembers *why* red-hair was considered popular 500 years ago, maybe it lasts in some folk story but it has no adaptive significance. It's another level of selection called sexual selection and it can have very profound but weird consequences. A couple of examples within our own species may be breast size in women (it isn't adaptive in the sense that women with larger breasts have more or healthier babies) or facial hair in men (which seems like an advertisement of virility). It may be that these are the kinds of traits that become 'fashionable' at some point in a species' evolutionary history and then become locked in. Both breast size and facial hair are perfect kinds of 'signals' for sexual selection because they are useless (and therefore costly) and hard to fake. The textbook example of this kind of selection is the tail of peacocks. They're big, expensive to build (in the sense of the calories that go into building that can't be used for building, say, better flight muscles) and make it very difficult for peacocks to fly. However, peahens prefer peacocks with large, brightly colored, garish plumage. It would be better for the peacocks to grow smaller tails that were still larger than their neighbors but then, that would mean that it would behoove a neighbor to have a slightly larger tail than that. Does it help the male survive? No. It does, however, help him spread more genes around because males with the best plumage have the best mating opportunities.

Because of medicine, with the exception of some disease resistance, I don't know that we are going to be selecting for physical traits as much as we are for mental traits. But it's hard to predict future evolution because nature is far more clever than we are and she knows what she likes.

Cheers
Aj