Slashdot, as usual, has a pretty good (not necessarily correct) discussion on Prof. Clark’s thesis
Also at Slashdot, they discuss a study that finds genetic diversity has decreased in England over the last 1700 years. You’ll notice the explanation for this change is the black death, an explanation Clark dismisses to explain the “rise of modern man”
gnxp also discusses the genetic diversity study. He points out while specific alleles may be becoming less diverse, there’s been a marked increase in the combinations of different genes (e.g. most Latino men have Spanish Y chromosomes but Indian mitochondrial DNA).