Brent wrote:
>Thanks for the input. I understand the difficulty with generating
>our modern genetic diversity from two individuals, or even a
>slightly larger bottleneck, in a relatively short time. Thanks for
>all the references Glen. But consider a hypothetical situation for
>a moment (I guess this is your weird scenario David). Suppose you
>have a population of early humans several thousand or even
>hundreds of thousands strong. The population is genetically
>diverse, and the diversity is fixed; we will not call on mutations
>as a source of new variation. Within that population two
>individuals are “given” an inheritable trait that is highly
>advantageous. After many generations the trait is found in roughly
>half of the population. Sexual selection may kick in and after
>many more generations individuals without the trait may be totally
>eliminated from the gene pool. At this point each individual in
>the population must have a genetic contribution from the first two
>to have the trait, even though no bottleneck has occurred. But
>they also have genetic contributions from myriad other lines and
>the population is still genetically diverse, even though no
>mutations have occurred. This way the genetic variation doesn’t
>have to arise in a short time. It was already there and never left.
>
One caution. Just being descended from someone doesn't mean you got any
genes from that person, which show how silly it is to claim descent from
ancient famous peoples. You have 46 chromosomes. You got 23 from your
mother and 23 from your father. By the time you go back to your
great-great-great-grandparents, you have 64 people, each trying to give you
a chromosome. 18 of the 64 people in your ancestry from that time didn't
give you a chromosome. Two generations prior to that, 210 of your 256
ancestors didn't give you a single chromosome. A recent article in Atlantic
Monthly points out that nearly everyone of European descent is descended
from British Royalty, Mohammed, and are descended from 80% of the
individuals alive in Europe merely 1000 years ago. But for most of those
Europeans living in 1000 AD we don't have any of the direct lineal copies of
their genes.
Now, occasionally cross over occurs--a process in which parts of one
chromosome switch with its partner allowing some of the genes from your
ancient ancestors to be passed down to you. But it isn't a guarentee that
cross-over took place in any given lineage. So, even if you have Adam and
Eve in your scenario, it doesn't mean that everyone would get the trait,
unless, as you suggest, strong selection killed all the others off. And
what it would really mean is that that one chromosome with that super-trait
would be the survivor.
glenn
see http://www.glenn.morton.btinternet.co.uk/dmd.htm
for lots of creation/evolution information
anthropology/geology/paleontology/theology\
personal stories of struggle
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