Is humanity's genetic diversity on the decline?

/. has a post with the title Humanity's Genetic Diversity on the Decline, drawing from a recent paper which found that the mtDNA haplotype diversity in England was lower than 1,000 years ago. The authors were surprised because of course one presumes England is more cosmopolitan today than in the past, and so there would be more diversity. As a resolution to their finds they suggest that demographic dynamics, such as the Black Death with resulted in the death of 1/2 of the English in 1 generation, as well as selection for mtDNA variants (i.e., a haplotype is linked toward some functional benefit which increases its frequency within the population). One might also note that England has gone through massive population growth, and this has not always been equitable. In other words, the demographic explosion of the last few centuries might have been biased toward particular regions or segments of ...