Re: [asa] promise trumps biology (accepting biological evolution for Adam)

From: David Campbell <pleuronaia@gmail.com>
Date: Mon Dec 15 2008 - 20:05:28 EST

> On how long it would take for the present genetic diversity to develop from
> a single couple: This depends not only on knowing the extent of that
> diversity but also knowing how rapidly genetic change took place in the
> past. I gather that there's some, but not unlimited, wiggle room here.

There are three major problems to pinning this down:
1. Calibration-tying the amount of mutational divergence in living or
recently living organisms into known dates. Problems include
precision and accuracy of dates from various calibration techniques
and how to link specific humans (or other species) to specific
calibration points. For example, we can measure the current genetic
diversity among native Americans, but there's debate about exactly
when they first got here, in how many groups, and how large those
groups were. Plus, there's the challenge of ensuring that someone
analyzed is pureblooded native American without any other ancestry.

2. Knowing the genetic and population processes-what are the patterns
of mutation? what sort of population sizes were there? were there
complicating factors such as bottlenecks? is the gene under a
particular evolutionary pressure to change or to not change? how many
copies are in one individual? (for example, the MHC gene shows a very
high level of variation in humans, with several different versions
similar to different versions in chimps. However, there are multiple
copies within a single individual, and the function of the gene is to
vary rapidly to guide immune system self-recognition and non-self
recognition, so very high variation and convergence are both
expected.)

3. Wild overuse of undercalibrated, overly simplistic models, such as
taking a rate developed for a different organism or different gene,
assuming a constant mutation rate, getting a date out of some other
molecular paper instead of researching the
paleontological/archaeological literature, etc. For example,
statistically responsible re-examination of a claim (based on a
molecular clock) that arthropods originate well into the Precambrian
showed that in reality all the data could say was that arthropods
probably originated no earlier than a few billion years before the Big
Bang. We already knew that. Of course, with decent calibration and
realistic models of evolution (requiring a lot more computer power),
you can do better than that.

In the specific case of Adam and Eve, you have to decide what time you
think they lived before you can judge how much genetic variation
probably existed in humans at that time. You also have to decide
whether non-Homo sapiens sapiens (Neanderthals, "hobbits", etc.)
should be within the populations under consideration.

There are so many different population genetics patterns that can
happen, and so much variation in evolutionary rates, that it can be
very difficult to pin something down precisely. It's far from clear
whether levels of genetic variation can really tell us much more than
the fossil record for dating events of interest in humans, since we
have a pretty good record.

-- 
Dr. David Campbell
425 Scientific Collections
University of Alabama
"I think of my happy condition, surrounded by acres of clams"
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Received on Mon Dec 15 20:06:02 2008

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