Jeff Otto
93-05-11 16:04:44
Noah's mutant ancestors

Most geneticists firmly believe that the amount of genetic variety that
we see today in the human population simply cannot be accounted for
assuming a starting population of six individuals 4000 years ago.  The
ICR and other proponents of scientific creationism claim that it can.
To answer this question, I did a little bit of investigation, and played
with some numbers.   The results follow:

According to the introductory biology text "Biology of Cells", by Curtis
and Barnes, 1989 on page 361, the human genome consists of approximately
3.5X10^9 base pairs.

Now that we have the genome size, we need to look at the level of
diversity present in the population.  I originally wanted to look at a
single well studied gene, but instead settled on RFLP; the tool used for
DNA fingerprinting.  According to the introductory molecular biology
text "Genes IV", by Lewin, 1990 on page 98, comparisons between two
individuals show variations at a frequency greater that 1 per thousand
bases.  This seems pretty high to me (I'd guess 1 per ten thousand), but
I am not a human geneticist.  Unfortunately no source was given as to
where he got the number, so I couldn't check up on it.  Regardless, at
the end of my argument, we can switch between his number and mine, and
see how well it fits with Noah and his mutant ancestors.

So, assuming a genome size of 3.5X10^9 and allelic differences at the
rate of 1 per thousand bases, this gives us 3.5X10^6 alleles per
individual (the term "alleles" is my usage here only to indicate regions
of differences, it does not imply coding regions of genes or regulatory
regions there of).

In Noah's time there were six individuals.  Assuming complete
heterozygosity we generate a total of 6 X 3.5X10^9 or 2.1X10^9 alleles.

Today, approximately 5X10^9 individuals live on this planet.  Assuming
complete heterozygosity we get 5X10^9 X 3.5X10^9 = 1.75X10^16 different
alleles.

If we subtract the diversity given to us from Noah we get:
1.75X10^16 - 2.1X10^9 = 1.7499999979X10^16 alleles.  Obviously, Noah
didn't give us much in the way of diversity, so from here on in, I will
round off to 1.75X10^16.

Population algorhythms has never been my strong suit, and I decided
against attempting it now.  However, we can make some ballpark
assumptions that will keep us in the right area.  First, I assume a
generation is roughly 15 years.  4000/15 gives us 267 generations.

1.75X10^16 alleles/267 generations gives us 6.55X10^13 alleles created
in each generation.  This assumes an equal burden on each generation,
which is obviously incorrect.  In reality as your population increases,
your ability to generate diversity increases; while the mutation rate
remains roughly constant.  But for the sake of argument, I will use
numbers that would be more favorable for creationist theory.

6.55X10^13 alleles per generation/5.0X10^9 individuals gives us 13,100
mutations/individual*generation.  Of course not everyone of the 5
billion people on this planet are currently capable of propagation, so
this number is inflated perhaps by as much as 75-80 percent.  Yet the
larger the population the better the chance that the creationist
argument can hold water; so I will leave it.  (If I was really nasty, I
would look at 6.55X10^13 alleles per generation/6 individuals; now that
would be a nasty mutation rate).

Now taking the genome size of 3.5X10^9 and dividing by 13,100 mutations
per individual*generation and we get one new allele generated for every
2.6X10^5 bases/generation.  Dividing by 15 years per generation we get
one new allele generated for every 1.78X10^5 bases per year.

What does science actually find?  Somewhere, I have a list of the
general mutation rates for various organisms.  I couldn't find it, so I
will use the number I am familiar with; for Drosophila we find base
changes at the rate of 1 change per 1X10^8 bases per year.  If I
remember correctly this number is a bit higher than for humans, but
since it would favor the creationist argument, I'll use it.

So lets compare rates
Noah gets 1 new allele for every 178,000 bases every year
Science finds 1 new allele for every 100,000,000 every year

Dividing 1X10^8/1.78X10^5 we find that Noah and his mutant decendants
chug out alleles at a rate 561 times faster than what science observes.
Quite frankly, a mutation rate this intense would be lethal.  Now, if we
assume that differences per individual are 1 in 10,000 bases rather than
1 in 1,000 we find a rate 56 times faster than what science observes.  A
rate in this ball park would be convienent for doing mutant screens in a
laboratory, like subjecting flies to EMS or X-rays.  Still, you are not
going to generate a healthy population.

Granted, the numbers here are soft, but keep in mind that at every step,
I have done what I could to give the creationists the benefit of the
doubt.  If someone would develop a nice population formula that we could
plug these numbers into, we could get a much more accurate estimate -
but the rate would most certainly be higher than what is given here.

Jeff