Date: Wed, 22 May 1996 16:15:04 -0600
Subject: Dr. Blood, evolution, genetic drift


>>> <glevy-AT-acnet.pratt.edu>  5/22/96, 02:57pm >>>
How long have blood samples been taken and classified? Unless one has
a sample of what we can agree was the "original" blood, how can we
say what the original vs. mutative (is that a real word?) blood is?
--------

There are a few other questions also.  _Everybody_ is "aboriginal" a
sense.  And how would aboriginals remain original O unless there is
something about non-O's such that non-O's were the ones who kept
moving away?  And how long have any of these populations been in any
one location anyway?  People have been in the western hemisphere for
possibly around 30,000 years.  All present populations
[pre-euro-invasion] could have moved around many many times since
then.  So I'm not confident about this particular story.  BUT...

The naive adaptationist approach is to say that the differences in
blood type frequencies between populations must be advantageous to
them, relevant to each of their different circumstances,
environments, locally endemic diseases, etc.  I know some work was
done to look for that, and some correlations claimed at one time. 
But I don't know of any good evidence that ABO types are actually
good for various different things.  

The mention of genetic drift reminds me that I was intending to get
back to Rahul's earlier comment on that [altho I'm not sure if I
recall his comment exactly].  There are a few ways to categorize
random changes in gene frequencies.  My definition of drift did not
make a separate category for "the founder effect".  As I understand
it, these are much the same thing.

In a large well-mixed population with random mating with respect to a
given gene or trait is no more likely to see an increase in any one
form of it, than it is to see a decrease.  If it confers no advantage
or harm relative to existing alternate forms, then its increase in
frequency or its disappearance are matters of chance.  The larger the
population, the less likely it is that for example all the brown-eyed
people just happen by accident to have less reproduction than the
rest.

Yet within a real-world smaller population, somewhat separate from
others, the smaller the population is the easier it is that by chance
gene frequencies will change from one generation to the next, and
perhaps in the same direction a few times in a row, which is all it
would take to possibly lose some genes entirely.

This is sometimes called "the founder effect" when a small number of
individuals separates from a larger population.  [This is also
implicated in the splitting of species, and may have a role to play
there sometimes.]  The separation could be caused by a river
deepening, a forest fire in between two areas, migration into new
territory, distance itself, etc.

A famous human example was a religious communal society in the US. 
It started very small, they moved out West, married only within the
society.  Within a few generations they had a very high rate of very
short-fingered people.  This is caused by a specific recessive gene. 
None of the founders had short fingers, but subsequent research
indicates that at least two of them were carriers of the recessive. 
Among their intermarrying and multiplying progeny, many people ended
up getting two copies of this gene and it was expressed.

Yet that gene had nothing to do with those people associating with
each other, adopting certain beliefs, forming a communal farming
society, etc.  It was pure accident, and those individuals are
actually less able at certain tasks than their long-fingered
neighbors.

Thus, it would be the grossest of "adaptationist" error to come along
and say 'Gee, there must be some reason why there is an advantage to
having short fingers in this situation, or else it wouldn't exist and
be so much more common in this population than in others!'

Good, real evolutionary biology seeks for the understanding that
allows us to see under what circumstances these kinds of things are
significant, and when and how much of a role they may have played in
evolution over all or in any particular case.  Understanding natural
selection and how it works is not to say that random changes never
happen or that everything is always "adaptive".  Reality is more
complicated than that, and scientists are trying to figure out how it
really works.

Lisa


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