he discusses in the next chapter, and mechanisms that prevent organisms from mating. These include geographic and ecological isolation. After two populations are changed sufficiently, "physiological" isolating mechanisms prevent them from mating, so they will remain distinct even if they are no longer physically isolated. This isolation is the end of a continuous process that includes division into races, and involves multiple mutations. Hybrid sterility In the chapter on isolating mechanisms, Dobzhansky deplores the "appallingly insufficient attention" that geneticists had devoted to isolation. The only isolating mechanism that they had studied was sterility in hybrid organisms, so he devotes the ninth chapter to a particularly detailed analysis of the literature on this subject. Experiments Through his work on Drosophila pseudoobscura, a species of fruitfly, Dobzhansky was able to identify that some populations of this species did not have identical sets of genes. Dobzhansky used experimental breeding in laboratories and gardens, and also surveys related to species in nature to help support aspects of organic evolution. The data in his book show the different genetic mutations and chromosomal changes that were observed. These experiments are vital to this book because they illustrate a transition from the laboratory to the greater field of genetics. Drosophila enabled researchers to gain a deeper understanding of how scientific knowledge about heredity has expanded and contributed to other areas of biology. By focusing on the opportunities and constraints of the research organism, persuasive accounts of the chromosomal genetics of Drosophila eventually evolved into the genetics of natural populations in the 1930s. All of the results from his experiments support the theory of modern evolutionary synthesis. Mutations Through his experiments, Dobzhansky discovers that mutation of genes leads to evolution within a specific species. Adaptations play a large role in genetic drift, and it is known that genes and mutations influence this genetic drift in a particular environment. Mutations can result from external influences within the surrounding environment, especially if an organism inhabits an area with harsh living conditions. An organism can adapt to its environment in order to better suit its own needs. When an organism successfully adapts, it has higher survival and reproduction rates. Therefore, there is a higher chance that its genes will be passed on to its offspring. Specific genes and alleles are then passed on to future generations to continue the trend of modern evolution that Dobzhansky presents in the book. Dobzhansky said