For most species, the difference is therefore much less than 5 percent. For Drosophila, the diversity per nucleotide site is about 1.5 percent. The genetic diversity of humans is considered low for a species, so most would fall in between 1/2000 and one or two percent in their genetic diversity. This is somewhat of a problem for the theory of evolution, because such genetic uniformity indicates that a species is young. This means that it went through a recent population bottleneck or recently originated due to a new mutation or isolation of a small group. If a species is old, and has not gone through a population bottleneck recently, its genetic diversity should be nearly 75 percent, assuming most of the DNA is non-functional. Even if half of the DNA is functional, the diversity at equilibrium should be at least (3/4)(1/2) or 37.5 percent. This indicates that all species today are far from equilibrium, since their genetic diversities are so much less than this. One can imagine that some species today are uniform because of their youth or due to a population bottleneck, but this does not explain why every single species should have such remarkable genetic uniformity. I also find it puzzling that this elementary fact of population genetics is never mentioned in any of the articles I have seen. However, this genetic uniformity of species is consistent with creationism, assuming that life is young and has not had much time to diversify since the original creation.
Peg Riley, an evolutionary biologist at Yale University, notes that while two humans might differ in 0.05% of their DNA, E.Coli strains vary by 5% -- "more diversity than you expect to find [in a single species]," she says.
Another study which even more strikingly reveals genetic uniformity among humans is found in Proc. Nat. Acad. Sci. vol. 93 Number 09 pp. 4360-4364 (1996), in which a 55 kilobase sequence of the human genome was examined and found to be identical in most individuals, with only a single nucleotide difference observed. The authors interpret this as evidence for a recent single origin for modern humans. Why the genetic diversity of this region should be less than one in 50,000 (actually, less than one in 100,000) and elsewhere about one in 2,000 is not clear. But this could indicate that the human race is much younger than biologists have assumed. For example, if one assumes a mutation rate of 2 x 10-8 per base pair per generation, which seems to be a popular figure among biologists (Science, 6 January 1995, pp. 35-36, for example), and a generation time of 20 years, then a diversity of one in 100,000 would be produced in 5000 years, assuming the ALU region is essentially neutral.
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