Hybridization is the process of interbreeding between individuals of different species (interspecific hybridization) or genetically divergent individuals from the same species (intraspecific hybridization). Offspring produced by hybridization may be fertile, partially fertile, or sterile.
Plants hybridize much more frequently and successfully than animals do. Pollen from flowering plants disperses widely and may land on flowers of other species. Plant forms are less stringently controlled than animal forms, and so the intermediate form of a plant hybrid is more likely to be physiologically successful.
One of the first persons to study plant hybridization was Josef Kölrueter, who published the results of his experiments on tobacco in 1760. Kölrueter concluded that interspecific hybridization in nature is rare unless humans disturb the habitat. Since that time, many instances of hybridization among various plant species have been documented.
Often interspecific hybrids are sterile or for some other reason cannot interbreed with the parental species. Occasionally sterile interspecific hybrids can undergo a doubling of their chromosome set and become fertile tetraploids (four sets of chromosomes). For example, the bread wheats that humans use today are a result of two hybridizations each followed by chromosome doubling to produce fertile hexaploids (six sets of chromosomes). In such instances the hybrids can become new species with characteristics different from either of the parents.
Interspecific Examples in Animals
Hybrids involving different species of the same genus are called interspecific hybridization (also called intra-generic). Common examples include Mule (male donkey x female horse), Hinny (male horse x female donkey), Liger (male lion x female tiger). Note that in mules and hinnies, the common genus the parents belong to is Equus and in liger, its Panthera. Other examples are zebra/donkey cross resulting in an offspring called zonkey, zebra/horse cross resulting in zorse, and zebra/donkey cross resulting in zonky. The offsprings from this cross could develop into adults but may not develop functional gametes. Sterility is often attributed to the different number of chromosomes the two species have, for example, donkeys have 62 chromosomes, horses have 64 chromosomes, etc.
Interspecific Examples in Plants
An interspecific hybrid is a cross between plants in two different species. Many times they will be from the same genus, but not always. The resulting plant may or may not be sterile.
Crop yields increase dramatically when hybridization is used to exceed one or more of the parents in size and reproductive potential. For example, boysenberries (Rubus ursinus x idaeus) were developed at Knott's Berry Farm in California. They are a result of a set of crosses between blackberries (Rubus fruticosus), European raspberries (Rubus idaeus) and loganberries (Rubus × loganobaccus).
Interestingly, Interspecific hybridization between a native and an invading plant species, or two invading species, sometimes results in a new, sexually reproducing species.