By Alan McGregor.
This essay is a short, brilliant explanation of the necessity for genetic isolation for speciation and general evolutionary progress.
One addition I would make is that - as long as original stocks are maintained - hybridization between (relatively) closely related stocks can, over time, result in stabilized blends that can represent new sub-species to be worked on by the forces of evolution. Here, for example, I mean intra-European crosses, or intra-East Asian crosses, etc. - NOT crossings between highly divergent stocks such as the major continental population groups.
Hybridization between related stocks can produce useful combinations of related gene complexes, increasing genetic diversity of the race, while avoiding the outbreeding depression and wide loss of genetic interests and parental kinship inherent in divergent crossings.
Other than that detail, the McGregor essay is excellent.
