Re: [asa] (fall) biological evolution and a literal Adam- logically inconsistent?

From: David Opderbeck <dopderbeck@gmail.com>
Date: Tue Sep 02 2008 - 19:50:32 EDT

David C. said: The presence of multiple MHC alleles in both humans and apes
does not
require a continuous human population containing each of those
alleles, because the MHC genes are under strogn selective pressure to
vary rapidly. Convergent arrival at the same allele is quite
possible, not to mention the multiple paralogous alleles in any one
individual.

I respond: David, is there some literature you could point to here? I've
always thought of this as a real problem, and my impression has been that
the literature is pretty settled otherwise. But I lack the background or
capacity to really evaluate it.

On Tue, Sep 2, 2008 at 6:28 PM, David Campbell <pleuronaia@gmail.com> wrote:

> A couple of posts raised issues of molecular clocks and what the
> mitochondrial Eve, etc. mean:
>
> > It is my understanding that MtDNA will not point likely point to an
> ultimate
> > first mother (ie Eve) but only to an "Eve" down a given MtDNA trail.
> > Confidence in this dating system seems fairly high. However, I get the
> > impression that the Y-chromosome method of dating is much less reliable.
> I
> > am especially weak in micorbiology, so, hopefully, someone here will
> address
> > this issue.
>
> > Another way around it is to find a mitochondrial Eve or a Y chromosome
> Adam.
> > I guess you can trace back mitochondrial DNA through women, back to
> "one"
> > about 150,000 years ago. The issue is that a similar process with men
> and
> > the Y chromosome only goes back about 50,000 years. So unless your
> > biological Adam and Eve lived 100,000 years apart, you end up with a bit
> of
> > a problem.
>
> What the mitochondrial "Eve", Y chromosome "Adam", etc. point to is
> the most recent common ancestor of all known modern versions. This
> will almost certainly be later than the actual time of origin of the
> group of interest. For example, suppose for the purpose of
> illustration that all living humans descend from Noah's family. All Y
> chromosomes would derive from Noah, makign the earliest possible date
> of divergence later than Adam. For the mitochondria, we don't have
> any evidence on the ancestry of Mrs. Shem, Ham, and Japeth, but if it
> traces back to a single female ancestor more recent than Eve then the
> divergence would be somewhat later. Furthermore, things can be lost
> later, too. If one of Noah's sons had only sons, (or if all his
> daughters had only sons, etc.), then that wife's mitochondrial lineage
> would be lost.
>
> Extrapolation of dates based on the degree of molecular difference can
> involve numerous assumptions, many of them incorrect. This is
> typically especially bad in
> the case of studies of molecular sequences in organisms. Dan Graur
> and William Martin (Reading the entrails of chickens: molecular
> timescales of evolution and the illusion of precision. TRENDS in
> Genetics Vol.20 No.2:80-86.) showed that more realistic assesment of
> the uncertainty in some popular molecular clocks do not allow you to
> draw more specific conclusions than that the organisms probably
> diverged from each other sometime between the Big Bang and now.
>
> There's a better data set for detailed studies on humans, and the
> extrapolation is not nearly as extreme, but still the uncertainty is
> probably much larger than typically reported. It also depends on
> whether a simplistic or a credible evolutionary model is used for
> calibration. Thus, I'm doubtful that one can confidently assert that
> Y chromosome Adam is younger than mitochondrial Eve based on the
> molecular data. However, such a result would be expected a priori as
> the higher variability in male than female fecundity (polygamy, etc.
> allowing for a higher maximum and conversely increasing the
> probability of other males having no reproduction) would tend to make
> male lineages more vulnerable to loss, increasing the chance of
> resetting the clock. Males also tend to disperse more, promoting
> homogenization of the male genetic lineage (e.g., the number of
> populations with no European component to the Y chromosome lineage is
> rather small).
>
> The presence of multiple MHC alleles in both humans and apes does not
> require a continuous human population containing each of those
> alleles, because the MHC genes are under strogn selective pressure to
> vary rapidly. Convergent arrival at the same allele is quite
> possible, not to mention the multiple paralogous alleles in any one
> individual.
>
> For Adam and Eve to be ancestral to all living humans, they could be
> either the last common ancestors of all living people or else
> ancestors of that ancestor. (The picture gets much more confused if
> you allow some genetic component from pre-adamites-i.e., Adam and Eve
> are in everyone's family tree but not the unique heads. Being _an_
> ancestor of everyone alive today is possible rather more recently than
> being _the_ ancestral pair). To be representatives, they would only
> need to be early enough to allow for whatever you make of the
> genealogies, plus time indicators based on any archaeologically
> detectable features that you think are uniquely associated with being
> spiritually human.
>
> --
> Dr. David Campbell
> 425 Scientific Collections
> University of Alabama
> "I think of my happy condition, surrounded by acres of clams"
>
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>

-- 
David W. Opderbeck
Associate Professor of Law
Seton Hall University Law School
Gibbons Institute of Law, Science & Technology
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Received on Tue Sep 2 19:51:06 2008

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