Souls are attracted to parents with whom they have karma to work out, previous associations, deep, intimate, wonderful relationships. There is no chance involved. There may even be a wide diversity in appearance. Evolution, thus, is the process by which the potentials of this divine essence may unfold. The animals have unfolded that which makes them animals, and so forth.
Human races are like streams.
The substance of the race is coming in and going out without pause. Yet the race retains its stamp, its marked characteristics. While the race may change slowly, rise to power and sink into obscurity, it retains a certain individuality. There is no such thing as a pure race. All the world's races are mixtures.