People keep asking me this question after the creationist event here in town — Mortenson spoke about how creationism is so much more egalitarian than evolution, and how the Bible talks about these wonderful things people did in the book of Genesis, like Cain going out and founding a whole city, by himself! At a time when the world population was 4, however, that doesn’t seem like a great accomplishment. Anyway, some people thought that far, realized that in the creationist conception of an entire world population arising from two people only, there was an obvious problem in the second generation.
Have no fear, the creationists already have an answer, as I explained before. Cain had sex with his sisters. The creationists are even proud of this explanation, and you can buy it on a postcard in their cheesy gift shop, which does make one wonder about their clientele a little bit.
The exhibit goes on to explain how this was OK, because Adam and Eve were perfect and carried no deleterious alleles that might have caused trouble when homozygous. They also chew us nonbelievers out for finding their stories a bit objectionable.
Since God is the One who defined marriage in the first place, God’s Word is the only standard for defining proper marriage. People who do not accept the Bible as their absolute authority have no basis for condemning someone like Cain marrying his sister.
And people who do not understand population genetics have no basis for arguing that a species can survive a population bottleneck of two.