Table 26.2. Reproductive Isolating Mechanisms Prezygotic Mechanisms (prevent fertilization and zygote formation) Geographic or ecological: The populations live in the same regions but occupy different habitats Seasonal or temporal: The populations live in the same regions but are sexually mature at different times Behavioral (only in animals): The populations are isolated by different and incompatible behavior before mating Mechanical: Cross-fertilization is prevented or restricted by differences in reproductive structures (genitalia in animals, flowers in plants) Physiological: Gametes fail to survive in alien reproductive tracts Postzygotic Mechanisms (fertilization takes place and hybrid zygotes are formed, but these are nonviable or give rise to weak or sterile hybrids) Hybrid nonviability or weakness Developmental hybrid sterility: Hybrids are sterile because gonads develop abnormally or meiosis breaks down before completion Segregational hybrid sterility: Hybrids are sterile because of abnormal segregation into gametes of whole chromosomes, chromosome segments, or combinations of genes F2 breakdown: F1 hybrids are normal, vigorous, and fertile, but the F2 contains many weak or sterile individuals Source: From G. Ledyard Stebbins, Processes of organic evolution, 3rd ed., copyright 1977, p. 143. Reprinted by permission of Prentice Hall, Upper Saddle River, NJ.