Huxleys line See Wallacea

HWRP See Hydrology and Water Resources Program.

hybrid An organism derived from crossing genetically dissimilar parents. Thus most individuals in an outbreeding population could be called hybrids. However, the term is usually reserved for the product of a cross between individuals that are markedly different. If two different species are crossed the offspring is often sterile, for example, the mule, which results from a cross between a horse and a donkey. The sterility results from the nonpairing of the chromosomes necessary for gamete formation. In plants this is sometimes overcome by the doubling of the chromosome number (see chromosome), giving a polyploid (see polyploidy), which may eventually give rise to a new species. By contrast, hybrids derived from different varieties of the same species are often more vigorous than their parents, and are selected and propagated by vegetative means by agriculturists and horticulturists. See chimera; hybrid vigor.

hybrid breakdown The situation in which a hybrid is fertile, but its offspring are not.

hybridization Processes that lead to the formation of a hybrid. Hybridization is common among angiosperm plant species, though often the offspring are partially or wholly sterile. In some the hybrids may be able to backcross with one or both parental species, leading to hybrid swarms. Artificial hybridization of normally self-pollinating species involves the transfer by hand of pollen from one plant to another, often using a paintbrush. Its success depends on the presence or absence of incompatibility systems that prevent successful fertilization between individuals of similar genetic makeup. Such techniques have been impor

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