breeding

In biology, the crossing and selection of animals and plants to change the characteristics of an existing breed or cultivar (variety), or to produce a new one. Selective breeding is breeding in which humans choose the parent plants or parent animals.

Ever since humans first carried out agriculture, they have been selecting which farm animals they allow to breed and which seeds to save from their crops to sow the following year. Plant and animal species show inherited variation. This is a result of the processes involved in sexual reproduction. Mutation also produces variation that is inherited. Selecting the parents used for breeding makes some variants more common, even though they may have been very rare originally. It can also bring together in one organism characteristics originally found in different organisms of the species.

If selective breeding is carried out over many generations this results in the production of varieties of plants and breeds of animals (see variety). For example ‘Cox's’, ‘Golden Delicious’, and ‘Bramley’ are different apple varieties. Cattle may be bred for increased meat or milk yield, sheep for thicker or finer wool, and horses for speed or stamina. Plants, such as wheat or maize, may be bred for disease resistance, heavier and more rapid cropping, and hardiness. Selective breeding of animals to produce new varieties helped Charles Darwin understand the effects of selection and understand its natural equivalent – natural selection, which can result in evolutionary change.

breeding

In nuclear physics, a process in a reactor in which more fissionable material is produced than is consumed in running the reactor.

For example, plutonium-239 can be made from the relatively plentiful (but nonfissile) uranium-238, or uranium-233 can be produced from thorium. The Pu-239 or U-233 can then be used to fuel other reactors.