Hi David and Glen
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.
Bivalve wrote:
Additionally, there is the paleontological evidence for human populations having been established in various regions for a long period of time. If we are looking for a common ancestor of all modern humans, it would need to be old enough to pre-date the dispersal.
I write:
Yes this is a problem. But as long as there is open gene flow or interbreeding between the dispersed regions a trait like this could be spread throughout the entire population, given enough time. But of course I havn’t given any time constraints for the scenario above. All I said was “many generations” and “given enough time”. It would be difficult to imagine a few thousand years being “enough time” considering the geographic extent of the dispersal.
Bivalve wrote:
One line of molecular evidence that would be difficult to reconcile with a single common ancestor of humans would be the presence of multiple alleles for a single DNA sequence shared between humans and other primates. If the variation was unlikely to have arisen convergently, this would suggest that multiple individuals carrying different alleles all contributed to our ancestry.
I write:
The hypothetical I gave is consistent with this since there was never a population bottleneck and multiple individuals carrying different alleles did contribute to the final gene pool. The original population is a product of evolution and will have genetic links with their ancestors. But of course I can’t give any evidence in favor of this, just a bunch of wishful thinking. Now, genetic variation since the flood...humans and animals... That’s a different can o’worms. I’m not touching that one :) Actually I think Glen’s last post on this thread addresses a recent bottleneck.
Brent
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