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Based on a literal reading of Genesis, most "young-earth" creationists (YEC's), believe that a violent global violent flood occured only a few thousand years ago, and extinguished all land life on earth except the pairs of animals and eight humans aboard Noah's Ark. However, this position contradicts extensive scientific evidence, and many Biblical scholars interpret the Flood as regional rather than global (since the Hebrew word for "land" can also mean "region" or "area" (Morton, 1997) or consider it an allegorical account.
Besides geologic evidence against a young earth and recent global flood, YECs and other anti-evolutionists have a problem explaining the vast range of modern diversity for both humans and other creatures, if they were reduced to such small populations only a few thousand years ago, or started from only 2 individuals of each "Kind" only several thousand years ago.
More specifically, they need to account for the many alleles (different states of a gene) found in modern populations today, if they did not come about through evolution. For example, a gene locus in the human leukocyte antigen complex has 59 different alleles (Ayala et al, 1993). However, each individual person normally has only two alleles for a given gene locus (one allele from each parent).
According to Genesis 7:7 the eight humans on the Ark consisted of Noah and his wife, their three sons and their sons wives. This means that that the Ark family had among them at most ten different alleles for each gene locus (3 x 2 = 6 for the wives, plus 2 each for Noah and his wife--their sons would have received all their alleles from their parents). Even if we allow the remote possibility that all three of Noah's sons were adopted and that all eight people on the ark were unrelated, each could have carried only 2 different alleles for each gene locus, and the entire ark family would have 2 x 8 = 16 alleles for each gene locus. So where did all the additional alleles in the modern population come from? In mainstream science, such alleles are the result of natural selection acting on mutations (sometimes involving duplicate genes as well as point mutations and "crossing-over"), acting over of millions of years. Strict creationists must account for them in only a few thousand years.
The problem becomes even worse when animal life is considered. According to Genesis 7: 2-3 Noah was commanded by God to take on the Ark seven of each kind of clean land animal and bird, and two of every unclean animal. Although this appears to contradict Genesis 7:8 which indicates that "pairs" of each clean and unclean animals entered the Ark, in order to allow the most alleles, let's assume the first version is correct. This means that for clean animals, at most fourteen alleles are represented, and for unclean animals, four alleles. Since each species has tens of thousands of genes, and since many genes involve many alleles (sometimes dozens), the question must be asked again: where did all the alleles come from? Some creationists try to explain how the millions of species of land creatures fit aboard the Ark by proposing that the Genesis "kinds" were broader than species, perhaps similar to genera or even families. However, this would only exacerbate the problem, as it would require even more alleles be generated (evolve?) after the Flood. Indeed, some genera consist of dozens or even hundreds of modern species, with scores of alleles for gene loci they shared in common. In other words, if a pair of animals representing a genus were taken on the ark, YECs would need to account for vast numbers of new alleles within many new species within a few thousand years.
The problem of genetic diversity in a YEC framework becomes even more severe when one considers that only about a thousand years before the Flood (as most YECs interpret Genesis), the entire human genome resided in two individuals: Adam and Eve. Each of them could have had at most 2 alleles per gene, or 4 alleles per gene between them. This would also be the case if each "kind" of land animal was created as a single pair. Although the Bible does not clarify this, YECs insist that only two original humans were created.
Creationists often claim that evolution cannot produce any new genetic information, but in view of the above considerations, many new alleles (which are clearly forms of new information) must have arisen by evolution. In fact, to account for the large numbers of alleles seen in current populations from the very limited ones at the presumed time of the Creation or Flood, would require more dramatic and rapid evolution than even "evolutionists" allow.
Ayala, F.J. et al. 1993. MHC polymorphism and human origins. Scientific American 269(6):78-83.
Morton, Glenn. 1997. Why the Flood is Not Global. Web article at: http://home.entouch.net/dmd/gflood.htm
Vuletic, Mark I., 2007, From "Defenders Guide to Science and Creationism" website at: http://www.vuletic.com/hume/cefec/4- 7.html
For more information on the inheritance of human eye colors, see: http://en.wikipedia.org/wiki/Eye_color
Genesis 7 (NIV):
1 The LORD then said to Noah, "Go into the ark, you and your whole family, because I have found you righteous in this generation. 2 Take with you seven [a] of every kind of clean animal, a male and its mate, and two of every kind of unclean animal, a male and its mate, 3 and also seven of every kind of bird, male and female, to keep their various kinds alive throughout the earth. 4 Seven days from now I will send rain on the earth for forty days and forty nights, and I will wipe from the face of the earth every living creature I have made." 5 And Noah did all that the LORD commanded him. 6 Noah was six hundred years old when the floodwaters came on the earth. 7 And Noah and his sons and his wife and his sons' wives entered the ark to escape the waters of the flood. 8 Pairs of clean and unclean animals, of birds and of all creatures that move along the ground, 9 male and female, came to Noah and entered the ark, as God had commanded Noah. 10 And after the seven days the floodwaters came on the earth. -------