Interfertility

In biology, a hybrid, or crossbreed, is the result of combining the qualities of two animals or plants of different breeds, varieties, species or genera through sexual reproduction. Hybrids are not always intermediates between their parents (such as in blending inheritance), but can show heterosis or hybrid vigour, often growing larger or taller than either parent. The concept of a hybrid is interpreted differently in regards to animal or plant breeding, where there is interest in the individual parentage; in genetics, where attention is paid especially to the numbers of chromosomes; and in taxonomy, where a key question is how closely related the parent species are, and it is rare for the parents to be as far apart as in different genera.

Species are reproductively isolated by strong barriers to hybridisation, which include morphological differences, differing times of fertility, mating behaviors and cues, and physiological rejection of sperm cells or the developing embryo. Some act before fertilization; others after it. 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. A few animal species and many plant species, however, are the result of hybrid speciation, including important crop plants such as wheat. Doubling the number of chromosomes to create polyploids is important in hybrid speciation, because homoploid hybrids (with the same number of chromosomes as the parent species) are rarely fertile; the polyploid hybrids are allopolyploids.

Human impact on the environment has resulted in an increase in the mixing of species, with introduced species worldwide, and this in turn has resulted in an increase in hybridization. The genetic mixing may threaten many species with extinction, while genetic erosion in crop plants may be damaging the gene pools of many species for future breeding. Many commercially useful fruits, flowers, garden herbs and trees have been produced by hybridization. One flower, Oenothera lamarckiana, was central to early genetics research into polyploidy.

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