Some time ago, someone mentioned that scientists had found there to be
positive selection occuring for those genes responsible for ADHD.
Here's the latest:
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YOUR CAVEMAN GRAMPS HAD ANTS IN HIS PANTS
THE PAPER Evidence of positive selection acting at the human dopamine
receptor D4 gene locus
THE JOURNAL Proceedings of the National Academy of Sciences, January 8,
2002
THE AUTHORS Yuan-Chun Ding, Deborah L. Grady, James M. Swanson, and
Robert K. Moyzis, et. al.
THE GIST A genetic mutation that causes hyperactivity could have been
crucial to human development.
BEFORE TRANSLATION "Based on an observed bias toward nonsynonymous amino
acid changes the unusual DNA sequence organization and disequilibrium
surrounding the DRD4 7R allele, we propose that this allele originated
as a rare mutational event that nevertheless increased to high
frequency in human populations by positive selection."
SCIENTISTS BELIEVE THAT some of the bad genes that we humans walk around
with today are there for a good reason. By "bad genes," they mean the
ones that predispose young people to disabling diseases and disorders;
by "good reason," they mean an evolution- ary advantage. If these genes
were entirely bad, say scientists, they would not be so common.
That's the spirit in which biologist Robert Moyzis and colleagues from
the University of California at Irvine studied attention deficit
hyperactivity disorder. The ailment is linked to a variation of a gene
called DRD4. This gene makes a receptor for dopamine, a biochemical that
in addition to con- veying feelings of pleasure, also helps people focus
on specific tasks. About
50 percent of kids diagnosed with ADHD have the gene variation, which is
present in about ic percent of the population worldwide. Moyzis won-
dered: How could a gene that makes it almost impossible for people to
con- centrate be so widespread? At some point during our evolution, he
decid- ed, the gene must have given its bear- ers a fantastic edge. But
what could the advantage have been?
Moyzis and his team decided to look at the gene closely, and what they
found surprised them. There are more than io different versions of the
DRD4 gene, but by far the oddest is the one associated with ADHD. By
analyzing its structure and comparing it with the other versions, they
con- cluded that the variation must have occurred suddenly, rather than
evolving over many years. What's more, says Moyzis, "the genes adjacent
to the variant DRD4 were almost exactly the same, whether in an
individual from Africa, Amer- ica, or Europe." Since genes constantly
rearrange themselves and mutate at a certain rate, an ancient variation
should have been sur- rounded by many different combinations of genes
among different people. Consequently, he reasoned, the gene must have
sprung up sometime in our recent history.
Moyzis then plugged this information into some equations and fig- ured
the gene variation likely arose and spread some 50,000 years ago-right
around the time that scientists estimate humans began migrating out of
Africa. Could the gene have made its bearers so antsy they couldn't help
but pick up and move to a new continent? Moyzis believes it's quite
possible. The gene is active in the frontal lobes of the brain-our
emotional control centers and home to our personalities. The frontal
lobes are especially important in problem solving and learning from
one's experience. This part of the brain is also responsi- ble for
traits like spontaneity. Scientists haven't yet figured out exact ly how
variations in the gene cause differences in a person's behavior. But
they do know that people who have the version associated with ADHD have
a penchant for thrill seeking. What's more, the version is rare in
Asians hut common in South Americans-evidence that it could have spread
as people migrated into Europe and the New World
There's another theory as to why this gene variation became sc common.
Maybe the hyperactive males-wild, crazy, unpredictable hunks that they
were-attracted women in large numbers, and still do today. Of course,
it's just a theory.-GuNJAN SINHA
--Walter Watts Tulsa Network Solutions, Inc.
"No one gets to see the Wizard! Not nobody! Not no how!"
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