This textbook, originally published in 1970, is a classic in the field of Population Genetics. It presents the field of population genetics, starting with elementary concepts and leading the reader well into the field. It is concerned mainly with population genetics in a strict sense and deals primarily with natural populations and less fully with the rather similar problems that arise in breading livestock and cultivated plans. The emphasis is on the behavior of genes and population attributes under natural selection where the most important measure is Darwinian fitness. This text is intended for graduate students and advanced undergraduates in genetics and population biology This book steers a middle course between completely verbal biological arguments and the rigor of the mathematician. The first two-thirds of the book do not require advanced mathematical background. An ordinary knowledge of calculus will suffice. The latter parts of the book, which deal with population stochastically, use more advanced methods. Chapter Titles: 1. Models of population growth. 2. Randomly mating populations. 3. Inbreeding. 4. Correlation between relatives and assertive mating. 5. Selection. 6. Populations in approximate equilibrium. 7. Properties of a finite population. 8. Stochastic processes in the change of gene frequencies. 9. Distribution of gene frequencies in populations. Appendix. Some statistical and mathematical methods frequently used in population genetics. Bibliography. Glossary. Index.