Selective Breeding

views updated May 17 2018

Selective Breeding

Selective breeding is evolution by human selection. As nineteenth-century British naturalist Charles Darwin noted in Variation of Animals and Plants under Domestication, selective breeding may be methodical or unconscious. Methodical selection is oriented toward a predetermined standard, whereas unconscious selection is the result of biases in the preservation of valuable individuals. Methodical selection requires great care in discriminating among organisms and is capable of rapid change in specific traits, such as milk production or silk color. Unconscious selection, more common in ancient times, resulted in grains and seeds such as wheat, barley, oats, peas, and beans, and in animal traits such as speed and intelligence.

Historical Overview

Selective breeding began about 10,000 years ago, after the end of the last Ice Age. Hunter-gatherers began to keep flocks and herds and to cultivate cereals and other plants. This process of domestication was probably stimulated by a combination of human population pressure and environmental stress caused by a rapid change in climate. Global warming at the end of the Ice Age created drought in areas where rainfall had previously provided sufficient water, forcing people to congregate around reliable water sources. The increased population density favored the cultivation of plant and animal species for use during times when they were not naturally plentiful.

Selective Breeding vs. Natural Selection

Like natural selection, selective breeding requires genetic variation on which to act. If the variation in a trait is strictly environmentally induced, then the selected variants will not be inherited by the next generation. Selective breeding also requires controlled mating. Thus, animals that are social and easily manipulated, such as bovids , sheep, and dogs, were easier targets for selective breeding than territorial species, such as cats and other carnivores. Cultures without a strong concept of property rights, such as those of pre-Columbian South America, were less likely to domesticate species because of their difficulty segregating different breeds. A short generation time also facilitates selective breeding by speeding up the response to selection. For example, most plants with multiple breeds (races) are annuals or biennials.

Selective breeding differs fundamentally from natural selection in that it favors alleles (forms of a gene) that do not contribute favorably to survival in the wild. Such alleles are usually recessive , for otherwise they would not persist in wild populations. Selective breeding is essentially a process of increasing the frequency of rare, recessive alleles to the point where they usually appear in homozygous form. Once the wild-type alleles are eliminated from the population, the process of domestication has become irreversible and the domestic species has become dependent on humans for its survival.

There is abundant evidence of the effectiveness of selective breeding. In general, there is more genetic variation among breeds of the same species for valuable traits than for others. For example, tubers are diverse among potatoes, bulbs are diverse among onions, fruits are diverse among melons. The implication is that selective breeding for valuable traits has created the diversity.

Domestication

The earliest archaeological evidence of selective breeding has been found in the Near East, where plants and animals were domesticated 10,000 years ago. China followed suit 2,000 years later, and sub-Saharan Africa, central Mexico, the central Andes, and eastern North America began selective breeding around 4,000 years ago. Historical evidence includes rules given for influencing sheep color in chapter thirty of Genesis, ancient Greek philosopher Plato's note that Glaucus selected dogs for the chase, Alexander the Great (356-323 B. C. E. ) selecting Indian cattle, Roman poet Virgil's (70-19 B. C. E. ) description of selecting the largest plant seeds, and the Roman emperor Charlemagne's selection of stallions in the ninth century. The Incas of Peru rounded up wild animals and selected the young and the strong for release, killing the rest. This strategy mimicked the action of natural selection, whereas elsewhere artificial selection used the most valuable individuals.

Selective breeding was invented independently in several different parts of the world, but its first appearance was in the Fertile Crescent, an alluvial plain between the Tigris and Euphrates Rivers. Ten thousand years ago, hunter-gatherers in a western part of the Crescent known as the Levantine Corridor began to cultivate three cereal crops: einkorn wheat, emmer wheat, and barley. Each was descended from a different wild species. A thousand years later, hunters in the eastern region of Zagros began to herd goats. Within 500 years after that, cereal cultivation and goat herding had spread to the center of the Crescent and combined with sheep and pig herding to form a diverse agricultural economy.

The process of domestication resembles a common mechanism of natural speciation. First, a barrier is created to separate a species into distinct reproductive groups within its geographical range. Over many generations the reproductively isolated groups begin to diverge as a result of selection, whether artificial or natural. All of a species' adaptations to artificial selection, both deliberate and incidental, are referred to as its "adaptive syndrome of domestication." Domesticated species eventually lose the ability to survive in the wild as part of their adaptive syndromes .

The path to domestication followed a stereotypical sequence of events. The first step in the domestication of a seed plant was the disturbance of the earth near settlements. This disturbed habitat facilitated the spread of pioneer plants that were adapted to natural disturbance. It also provided a colonization opportunity for the plants with seeds gathered, and dropped, by humans. The second step was the deliberate planting of seeds that were gathered from favored plants in the previous generation. Favored species tended to be pioneers adapted to growing in dense stands. One byproduct of this process was the selection of greater harvest yields. Since seeds that were collected were the only ones that reproduced, selection for increased seed production was strong. Another hallmark of domesticated plants was rapid sprouting, since competition between seedlings can be strong. The seeds themselves lost the ability to lie dormant and become larger. All of these traits were accidental byproducts of the storage and planting of seeds, rather than the results of methodical selection.

The domestication of animals also produced stereotypical traits. Animal species that were hardy, useful to humans, easy to breed in captivity, and friendly toward humans and each other tended to be successful targets for selective breeding. Of particular importance was flexibility of feeding habits, which facilitated human management. Solitary species with idiosyncratic feeding behaviors were unlikely to reproduce successfully in captivity in spite of early agriculturalists' best efforts. Domestic animals were probably already an important source of food before they were domesticated. Just as plants became domesticated as a result of controlled reproduction, animals were reproductively isolated as herds and flocks. There is, however, only one hallmark of selective breeding in animals: small body size. The remainder of the adaptive syndrome of domestication is unique to each species.

see also Domestic Animals; Evolution; Farming; Genetics; Genetic Variation in a Population; Natural Selection.

Brian R. West