selective breeding, the practice of mating individuals with desired traits as a means of increasing the frequency of those traits in a population. In selective breeding, the breeder attempts to isolate andpropagate thegenotypes (genetic constitutions) that are responsible for an organism’s desired qualities in a suitableenvironment. Such qualities generally are economically or aesthetically desirable to humans rather than useful to the organism in its natural environment.

Selective breeding results from the inheritance ofgene-associated traits and, more specifically, from changes in the frequencies with which desired traits occur in populations. The latter in turn alters gene frequencies in the population. Sinceevolution is based on shifts in gene frequency, selective breeding is considered to be a driver of evolution.

Historical developments and examples

Humans have been modifying the traits ofplant andanimal species through selectivebreeding for thousands of years. More than 9,000 years ago in Mesoamerica, for example, humans began selectively breedingteosinte plants that had greater numbers of kernels, and this practice eventually gave rise tocorn (maize;Zea mays), which is today one of the most widely distributed of the world’s food crops. Many other plants that are used by humans have undergone similarselection processes. Cruciferous vegetables, examples of which includekale,broccoli, andcauliflower, share a common ancestor: wildmustard (Brassica). Selection of wild mustard for specific traits, beginning at least 2,000 years ago, led to the emergence of different versions, or cultivars, of the species. Selection for wild mustard plants with large leaves, for example, resulted in kale, whereas selection for enlarged flower buds led to cauliflower. Likewise, domesticdogs (Canis lupus familiaris) first emerged possibly as early as 30,000 years ago in Southeast Asia—and more certainly between 29,000 and 14,000 years ago in Eurasia—as a result of selective breeding ofwolves with companionable traits. Since then, selection for fur colour and fur thickness and length, as well as for body type and temperament, has resulted in more than 400 distinct breeds of dogs.

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animal behaviour: Artificial selection

Scientific understanding of selective breeding expanded particularly in the 19th century, when English naturalistCharles Darwin used his experience with selective breeding of captivepigeons to form his insights aboutnatural selection and evolution. Beginning in the second half of the 20th century, advances in genetictechnology enabled researchers to modify traits across generations of organisms by introducing novel genetic variations, rather than by breeding. True selective breeding, through the mating of individuals, however, continues to be a valuable tool in research and agriculture. In particular, it has provided critical insight into mechanisms of inheritance, genetic disease, and geneticdiversity. For example, selection has been so intense in somedog breeds as to actually reduce genetic diversity in small breeding populations, which hasmanifested in various ways, including increased risk of disease and decreased survival. These effects are strikingly evident inFrench Bulldogs, which have been selected for compact features, resulting in narrow nostrils and a flat face, causing the animals to experience breathing problems that affect theirquality of life and survival.

Heritability of traits

Heritable traits become more frequent in populations at varying rates. The ease with which atrait becomes more frequent across generations is referred to asheritability. Scientists can measure the heritability of distinct traits to help breeders estimate how long it will take for a trait to increase in frequency within a population. In some instances, traits are determined by a single gene. This occurs when thealleles (any of one or more genes that can occur at the same site on achromosome) have a dominant/recessive relationship, wherein both alleles affect the sameinherited characteristic but one allele is expressed in an observable manner over the other. Dominant traits are highly heritable, and their frequency in a population can be changed over the course of a single generation. In other cases, multiple genes contribute to the expression of a trait. Often, the trait is also partly influenced by the environment. In such cases, selective breeding over many generations is required to dramatically change the frequency of the trait in a population.