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From: roy@phri.UUCP (Roy Smith)
Newsgroups: net.origins
Subject: Re: So simple, even a creationist can understand!
Message-ID: <327@phri.UUCP>
Date: Sun, 14-Jul-85 22:38:07 EDT
Article-I.D.: phri.327
Posted: Sun Jul 14 22:38:07 1985
Date-Received: Sat, 20-Jul-85 02:48:14 EDT
References: <2156@ut-sally.UUCP> <347@scgvaxd.UUCP>
Organization: Public Health Research Inst. (NY, NY)
Lines: 66
Keywords: Natural Selection

> *How* do the beneficial mutations get amplified by Natural Selection?

	A perfectly reasonable question.

	Let's suppose that due to a single nucleotide being wrong in my
DNA, the hemoglobin in my blood tends to crystalize when it shouldn't
(we're talking sickle cell anemia, here).  I'm weaker, slower, etc. than
you are.  I'm less likely to survive to puberty than you are because I
can't compete for food or run away from attackers as well as you can.
Even if I reach sexual maturity, I'm less likely to father children (and
pass on my mutated DNA to my progeny) than you are because when we fight
over a woman, you are likely to win.  Thus, this random mutation gets
*selected against*.

	I can't think of such a striking example in the other direction,
but the idea is the same.  Suppose a random mutation causes my hearing to
be slightly better than yours in some way.  I'm more likely to hear an
attacker in time to save myself than you are.  When hunting, I'm better
able to tell where my prey is.  In this case, the effect is small (as it
is with most beneficial mutations).  However small the advantage is, I am
more likely to propagate my DNA than you are.  This mutation gets
*selected for*.

	Actually, the vast majority of mutations are harmful.  This
doesn't change the basic idea, though.  The harmful ones get weeded out
because they either don't survive or don't reproduce as well as the
wild-type (non-mutated) organisim.

	When talking about mutations, one needs to make a differentiation
between phenotype and genotype.  Phenotype refers to the observable
characteristics (the phenomena you can measure) of a system.  Genotype
refers to the genetic makeup; the two are only loosely related.  Small
changes in genotype can produce large changes in phenotype (as in sickle
cell anemia).  Conversely, large changes in genotype can produce little or
no phenotypical changes.

	Mutations are changes in the genotype.  Evolutionary pressure is
applied, however, because of changes in phenotype.  The single base change
mentioned earlier is a genotypical change.  This genotypical change
produces the sickle cell phenotype.  Which brings us back to what appears
to be a contradiction.  If sickle cell anemia is bad for you, why *hasn't*
natural selection selected it right out of existence?

	You have two sets of genes which make hemoglobin, one given to you
by your mother, the other by your father.  The sickle cell phenotype is
recessive.  That means that you only exhibit the symptoms if *both* of
your copies of the gene are defective.  If only one copy has the sickle
cell genotype, you are said to the have sickle cell trait; you can pass it
on to your children but you don't have the symptoms yourself.

	Well, it turns out that having sickle cell trait is actually
beneficial.  If you have the trait, you make both the good and the bad
kinds of hemoglobin.  It turns out that this is sufficient to make your
red blood cells function properly and you don't get sick.  However, the
parasite that causes malaria lives in your red blood cells, and *it* can't
tolerate the mixture of good and bad hemoglobin.

	So we have an interesting situation.  People with sickle cell
trait have protection against an often fatal disease so the trait gets
selected *for*.  On the other hand, two people with sickle cell trait have
a 25% change of their children having the anemia, so it gets selected
*against*.  Which of these factors stronger?  Check back with me in a few
million years and I'll let you know.
-- 
allegra!phri!roy (Roy Smith)
System Administrator, Public Health Research Institute