that natural mutation, aided by variation, can lead to change when acted upon by natural selection. Mutations were thought to be relatively rare and other variations were even considered to be harmful. Since an organism's overall genetic make-up was the result of natural selection, with damaging mutations weeded out, wild populations were assumed to have very few mutations. As a result, evolution was said to be a relatively slow process. One of Dobzhansky's major contributions in this book was to show that this view of slow-moving evolution was incorrect. While analyzing chromosome structure in wild populations of Drosophila pseudoobscura, Dobzhansky discovered a surprising amount of undiscovered variability. These variations could not be observed in the outward appearance of the individual organisms. Dobzhansky suggested that the preservation of extensive variation would allow populations to evolve rapidly as environmental conditions change. This book was a landmark in the evolutionary synthesis, as it presented the union of Mendelian genetics and Darwinian theory. In Genetics and the Origin of Species, polyploidy is considered as a type of mutation. Polyploid cells have a chromosome number that is more than twice the haploid number. The effects of polyploidy between two different species causes hybridization and even greater evolution. Natural selection and speciation Natural selection in an environment produces reproductive success, which benefits the species. Speciation is a process of evolution through which new biological species are formed. Dobzhansky studied the specifics of sexual, physiological, and behavioral isolating mechanisms in Drosophila pseudoobscura and Drosophila paulistrorum. Like many of his other studies, Dobzhansky's work on reproductive isolation was aimed at studying the process of evolution in action. Bringing samples from each population back into the laboratory, Dobzhansky showed that he could vary environmental conditions so as to produce the same changes in frequency of inversion patterns that were observed with changing seasons in the field. Dobzhansky concluded that such seasonal fluctuations were the result of natural selection at work, with temperature acting as the selecting agent. These masterful studies provided concrete support for the theory of natural selection, at the same time illustrating the fruitfulness of combining field and laboratory work in the study of evolution. Adaptive evolution occurs through the dominance and survival of competing genes within a species. This is caused by increasing the frequency of those alleles whose phenotypic effects selfishly promote their own reproduction. He also believed new species could not arise from single