BIOLOGY Volume 3 - A Guide to General Biology - 2004

26. EVOLUTION, OR THE HISTORY OF LIFE ON EARTH

26.7. Evidence for Evolution

26.7.4. Plant and Animal Breeding

One of the earliest achievements of human civilization was the derivation of plant varieties and domestic animal breeds from wild ancestors. By selecting individuals possessing desirable variations—such as larger size, or a more pleasant taste and aroma—humans preserved these traits through artificial propagation using selective breeding or pollination. As a result of continuous Selection, humans developed the breeds of domestic livestock and varieties of cultivated crops that we have today. Archaeological findings show that early humans demonstrated considerable skill in breeding cattle, pigs, and poultry, as well as in cultivating cereal grains and certain vegetables. Before Mendel's work became known, the THEORETICAL FOUNDATIONS OF heredity and selection remained unclear, yet this did not limit practical human efforts. Translated into genetic terminology, humans preserve the genes desirable for achieving their goals and eliminate those that are unsuitable. In carrying out selection, they utilize the genetic variation existing in nature, as well as random Mutations that arise from time to time.

Crop and livestock breeders devote the greatest attention to developing economically valuable animal breeds (Fig. 26.9) and plant varieties, although they also create various breeds of dogs, cats, birds, fish, and numerous varieties of ornamental plants. Some Examples of phenotypic traits subjected to artificial selection are listed in Table 26.4. A new form of artificial selection has recently emerged—unintentional selection for resistance to Antibiotics, pesticides, and herbicides, affecting pathogens, agricultural pests, and weeds, respectively. This creates a vicious circle: the ever-increasing number of chemicals designed to combat harmful organisms leads to the appearance of new strains resistant to these agents.

Darwin used humanity's ability to "create" traits through directional selection, such as in the breeding of dogs and pigeons, as an argument that species could arise under natural conditions through an analogous mechanism. He suggested that in nature, the environment acts as the agent of natural selection instead of humans. Forms produced by artificial selection probably could not arise in nature, as in most cases they would prove unable to successfully compete with closely related wild forms.

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Fig. 26.9. The results of selection in animal husbandry. From the wild boar (A)—a native inhabitant of Europe, Asia, and Africa—many domestic breeds were developed through selection, including the British Large White, which yields excellent meat (B).

Table 26.4. Some phenotypic traits subject to breeding