Having established that the sources of natural variation are mutations and chromosomal rearrangements, Dobzhansky considers what shapes and preserves this variation. In the first edition of this book, he considers genetic drift as important as natural selection. As an example, he argued that drift was the reason for racial variations that were observed in Partula, a land snail, so these variations had no adaptive advantage. He argued that most such variations, which he called microgeographic race, was nonadaptive. Because variation is not always controlled by selection, we need to know the size of a population before we can predict its evolutionary dynamics.
