Fusion of male and female sex cells from different individuals of the same species. Cross-fertilization is necessary in animal and plant species that have male and female organs on separate individuals. Methods of cross-fertilization are diverse in animals. Among most species that breed in water, the males and females shed their sex cells into the water, where fertilization takes place outside the body. Among land breeders, fertilization is internal, with the sperm being introduced into the body of the female. By recombining genetic material from two parents, cross-fertilization maintains a greater range of variability for natural selection to act on, thereby increasing the capacity of a species to adapt to environmental change. See alsoself-fertilization.