I'd be interested, but evolution over the past 20,000 years doesn't affect the argument I'm making here, which looks at a long-term trend in evolution.

ADDED: There are some factors that will increase genetic exchange and selective pressure, as discussed in some comments below; but not that increasing genetic exchange often slows evolution. There's a balance between being able to spread beneficial mutations, and reaching premature convergence; the "sweet spot" of that balance is with very small communities, much, much smaller than continent-sized. Some equations and data indicate that species diversity is much larger when the environment is fragmented into small areas with little communication (google "island theory of biogeography").