Hybridization is the crossbreeding between different species of organisms. The term hybrid has also been used for crosses between varieties within a species (as in hybrid corn within the species Zea mays). The offspring with many heterozygous traits produced by intraspecific crosses may produce hybrid vigor. However, evolutionary biologists generally restrict the term hybrid to crosses between species.

According to the biological species concept, distinct species do not normally produce fertile offspring. This is most obviously the case with crosses between donkeys and horses, resulting in mules, which are very strong but are sterile. Crosses between lions and tigers (such as ligers and tigons) have also occurred in zoos. It is possible that the elusive "marozi" of Africa is a naturally occurring hybrid between leopards and lions. Many other hybridizations are possible between feline species. While even fertile hybrids may be produced between different feline species, they are rare in the wild, which suggests that either the species nearly always avoid one another when mating and/or the hybrid offspring have reduced health or fertility. Hybrids have also been reported between species of fishes. The bluegill (Lepomis macrochirus) can cross with the green sunfish (L. cyanellis). Intergeneric hybrids, such as between pumas and leopards, and between dromedaries and llamas, have also been reported.

Hybrids between plant species in the same genus occur more often than hybrids between animal species. . .

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