Just a quick follow up to my post about genetic screening of embryos and subsequent implantation. The spontaneous abortion rate for humans is very high. Probably on the order of 50% of fertilized ova implant and complete to term. I’ve seen numbers all over the place. In any case, I assume many of these are chromosomal abnormalities. But I’ve also posted to data which strongly suggests that immunological incompatibilities between mother & fetus also play a role in spontaneous abortions and may result in natural selection which we’re not too well aware of. In The Cooperative Gene the evolutionary biologist Mark Ridley suggests that because of spontaneous abortion we should be less than worried about reduced efficacy of natural selection in purging deleterious alleles from the gene pool; as the genetically unfit reach viability and reproduce their alleles will be culled at the stage of of the embryo and fetus.

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