[Revised entry by Samir Okasha on November 24, 2022. Changes to: Main text, Bibliography] Population genetics is a field of biology that studies the genetic composition of biological populations, and the changes in genetic composition that result from the operation of various factors, including natural selection. Population geneticists develop abstract mathematical models of gene frequency dynamics, extract predictions about the likely patterns of genetic variation in actual populations, and test the predictions against empirical data. A number of the more robust generalizations to emerge from population-genetic analysis are...