Mythological hybrids appear in human culture in forms as diverse as the Minotaur, blends of animals, humans and mythical beasts such as centaurs and sphinxes, and the Nephilim of the Biblical apocrypha described as the wicked sons of fallen angels and attractive women. For example, Neanderthals and anatomically modern humans are thought to have interbred as recently as 40,000 years ago. Human selective breeding of domesticated animals and plants has resulted in the development of distinct breeds (usually called cultivars in reference to plants) crossbreeds between them (without any wild stock) are sometimes also imprecisely referred to as "hybrids". It is also more occasionally done in the livestock and pet trades some well-known wild × domestic hybrids are beefalo and wolfdogs. One such flower, Oenothera lamarckiana, was central to early genetics research into mutationism and polyploidy. This is common in both traditional horticulture and modern agriculture many commercially useful fruits, flowers, garden herbs, and trees have been produced by hybridisation. A form of often intentional human-mediated hybridisation is the crossing of wild and domesticated species. This genetic mixing may threaten many species with extinction, while genetic erosion from monoculture in crop plants may be damaging the gene pools of many species for future breeding. Human impact on the environment has resulted in an increase in the interbreeding between regional species, and the proliferation of introduced species worldwide has also resulted in an increase in hybridisation.
A few animal species and many plant species, however, are the result of hybrid speciation, including important crop plants such as wheat, where the number of chromosomes has been doubled. Similar barriers exist in plants, with differences in flowering times, pollen vectors, inhibition of pollen tube growth, somatoplastic sterility, cytoplasmic-genic male sterility and the structure of the chromosomes. Some act before fertilization and others after it. Species are reproductively isolated by strong barriers to hybridisation, which include genetic and morphological differences, differing times of fertility, mating behaviors and cues, and physiological rejection of sperm cells or the developing embryo. In taxonomy, a key question is how closely related the parent species are. In genetics, attention is focused on the numbers of chromosomes. The concept of a hybrid is interpreted differently in animal and plant breeding, where there is interest in the individual parentage. Hybrids are not always intermediates between their parents (such as in blending inheritance), but can show hybrid vigor, sometimes growing larger or taller than either parent. In biology, a hybrid is the offspring resulting from combining the qualities of two organisms of different breeds, varieties, species or genera through sexual reproduction. Mules are smaller than horses but stronger than donkeys, making them useful as pack animals. A mule is a sterile hybrid of a male donkey and a female horse.
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