segment, is the basis for differentiation in Drosophila. He also points out that these effects demonstrate that the chromosome parts are interdependent. Variation in natural populations 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. Selection The sixth chapter discusses the evidence for natural selection from experiments in the laboratory and observations of nature. He considered examples of camouflage such as industrial melanism, in which moths and other arthropods evolve darker pigmentation when exposed to an environment with a lot of soot in it. He refuted some Mendelian misconceptions about the effectiveness of natural selection, but he also rejected the strict selectionism of Fisher. Polyploidy In the seventh chapter, Dobzhansky discusses polyploidy, a condition (common in plants) where an organism has more than two complete sets of chromosomes. (Humans are diploid, having one set each from the mother and father.) He discusses case histories such as that of Raphanobrassica, a hybrid between the radish and the cabbage. This is an example of "cataclysmic" speciation, an exception to his general rule that speciation is a slow process. Isolating mechanisms An isolating mechanism prevents members of two different species from successfully producing offspring. Dobzhansky invented the term, and in the eighth chapter he discusses their role in speciation. His views were sufficiently original for him to publish them in a journal article as well. Although previous authors had recognized the importance of isolation, their reasons were entirely different from his. George Romanes thought that isolation was a cause of change, whereas Dobzhansky sees two competing effects. Isolation reduces the rate of formation of harmful gene combinations, but it also restricts the scope of genetic variation. Thus, a species remains near a given peak in the fitness landscape and does not find new adaptive peaks. Dobzhansky presents a classification of isolating mechanisms. The main division is between hybrid sterility, which