Jeff Otto 930511 16:04:44 Noah's mutant ancestors Most geneticists firmly believe that the

Master Index Current Directory Index Go to SkepticTank Go to Human Rights activist Keith Henson Go to Scientology cult

Skeptic Tank!

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


E-Mail Fredric L. Rice / The Skeptic Tank