Received: by alpheratz.cpm.aca.mmu.ac.uk id DAA04736 (8.6.9/5.3[ref pg@gmsl.co.uk] for cpm.aca.mmu.ac.uk from fmb-majordomo@mmu.ac.uk); Wed, 31 May 2000 03:05:38 +0100 Message-ID: <002901bfcaa4$75c734e0$3b2484d8@default> From: "Anne" <tazzie@bolian.upnaway.com> To: <memetics@mmu.ac.uk> Subject: Re-Jabbering Date: Wed, 31 May 2000 10:03:16 +0800 Content-Type: text/plain; charset="Windows-1252" Content-Transfer-Encoding: 7bit X-Priority: 3 X-MSMail-Priority: Normal X-Mailer: Microsoft Outlook Express 5.00.2919.6600 X-MimeOLE: Produced By Microsoft MimeOLE V5.00.2919.6600 Sender: fmb-majordomo@mmu.ac.uk Precedence: bulk Reply-To: memetics@mmu.ac.uk
I came across this information at:
http://www.feedmag.com/feature/fr335_master.html
WHAT A DIFFERENCE A GENE MAKES
WHEN WE HEAR that two things are ninety-eight percent identical, we usually
assume that what is being measured is a single number, like the difference
between ninety-eight and a hundred bottles of beer on the wall. The 1.6
percent difference between humans and chimp DNA, however, doesn't measure
anything like that; it simply denotes that, in 1.6 percent of the places
where the genetic code of chimps and humans could differ, it does differ.
Human genes have something a bit over thirty million active units, called
nucleotides, which can be any one of the four chemicals, or bases, labelled
A, C, G, and T, that make up DNA. With approximately thirty million
nucleotides, 1.6 percent accounts for roughly 500,000 points of difference
between the average human genome and the average chimp genome. Furthermore,
because there are four bases, the difference is not a simple one: When
genetic material differs in a single spot, it can be one of four possible
values (A, C, G, T); when it differs in two spots, it can be one of four
times four, or sixteen, values (AA, AC, AG ... TT); three places can have
four times four times four, or sixty-four different values (AAA, AAC ...
TTT), and so on. (Actually, strands of DNA can differ in more complex ways,
with skips, repeats, and reversals, but this four-part difference is a good
approximation of genetic variation.)
This exponential power of difference means that if every one of these
500,000 nucleotides on the chimp genome were to spontaneously (and
miraculously) mutate in a single generation, there is not only no guarantee
that it would mutate into human genetic material, there is a guarantee that
it won't do any such thing, since the mathematical "space" between the human
and chimp genome is 4 values randomly accumulated over half a million
different spots, or four times four 500,000 times, roughly ten followed by a
hundred thousand zeros. This is a number so large there isn't even a name
for it, a number unfathomably much larger than the total number of atoms in
the known universe.
This is the ineluctable and one-way power of genetic difference between
animal species: In the seven million years since humans and chimps began
their separate evolution from a common ancestor, our DNA has drifted apart
in 500,000 places. But while animal species drift apart randomly, they never
drift together randomly. (Plants on the other hand, do sometimes
spontaneously hybridize.) You could make an analogy with a skydiver: In the
first second after a jump, a skydiver will have fallen only sixteen feet
from the plane, and you could correctly point out that the gap between the
skydiver and the plane is small. The essential fact about that gap, however,
is not size but irreversibility; no matter how small the distance separating
them, skydivers do not fall up into planes. Likewise, species can drift
apart, and one could argue that shortly after this separation starts that
they are still genetically close, but no matter how close they are, they
will never reclose the gap between them, because random genetic drift
introduces difference, not similarity. Once the ancestors of chimps and
humans stopped the interbreeding that would have kept this drift in check,
they became separated by the mathematical rachet of accumulated evolutionary
change. Rhetoric that equates the genetic similarity between humans and
chimps with identity vastly underestimates the one-way nature of the genetic
separation between even closely related species.
Even worse news for the Identity camp, the difference in DNA is not purely
mathematical, but biological as well, and biological difference cannot be
adequately described in linear terms -- "differs in 1.6 percent of DNA
bases" does not mean anything like "1.6 percent different." To see why this
is so, consider genetic differences much closer to home -- the differences
between men and women. Every member of the human race, male or female, has
twenty-three pairs of chromosomes, the repositories of genetic material. And
between men and women, forty-five of these forty-six chromosomes are
identical. It is only the forty-sixth chromosome which differs, and it
determines the gender of its host. When this chromosome is "X," its bearer
is female (the default gender); when it is "Y," its bearer is male. If the
Identity Theory of genetic difference is correct, we would expect men and
women to be almost indistinguishable from one another, and for whatever
miniscule differences that exist to be accounted for by the few genes
present on the forty-sixth chromosome, the only place where male and female
genetic material diverges.
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