genitalia tract to the other species' male sperm; 2) developmental isolation, the state in which the development of embryos resulting from cross-fertilization is irregular and ceases well before birth. Mechanisms that prevent hybrids from passing on their genetic material include: 1) hybrid inviolability, the state in which hybrids are weak, malformed, and die before being able to reproduce; 2) hybrid sterility; and 3) selective hybrid elimination, the state in which hybrids are less rigorous than parental forms and are selected against.
