1.2 Artificial selection
Selection acts on phenotypic characters whatever their origin, and can retain or eliminate the characters' genetic basis. Artificial selection is any selective breeding intentionally practiced by humans leading to the evolution of domesticated organisms. Artificial selection may oppose or amplify or be neutral in relation to natural selection. Most livestock, including dogs, cats, goats, pigs, cattle, sheep, guinea pigs, horses, geese and poultry and scores of crop plants were domesticated more than 5000 years ago. (Estimates of dates of domestication based upon the archaeological record differ enormously from those based on genetic analysis for almost all crops and livestock. However, experts agree that dogs were among the first mammalian species to become domesticated, followed by sheep.) During the last few hundred years, people have domesticated a wide range of other animals such as guppies, hamsters, mice and budgerigars, mostly for use as pets, together with thousands of culinary, medicinal and ornamental plants.
Pets and livestock are protected from predators and provided with food and shelter.
How would these living conditions alter the action of natural selection in domesticated populations?
Natural selection would be much weakened, particularly for the characters such as ability to recognise and escape predators, finding, eating and digesting food, and tolerance of severe weather conditions.
Name a wild species in which relaxation of selection for the ability to escape predators has happened naturally. How did the animals' behaviour differ from that of related populations that are exposed to predators?
Svalbard reindeer have had no natural predators for many thousands of years. Like domesticated cattle, these reindeer have lost the ability and the inclination to run away when approached.
However, unlike many domesticated animals, Svalbard reindeer retain the capacity to forage and breed in a difficult climate. This example emphasises the similarities between evolution in artificial or natural environments, driven by artificial or natural selection.
On what kind of characters would natural selection continue to act in populations of domesticated livestock?
Characters that affect fecundity such as age at maturity, litter size and parental behaviour.
Would domesticated livestock generally produce more or fewer offspring than their wild relations?
More, because human protection would prevent natural selection against traits such as heavily gravid mothers becoming unable to escape predators or find enough food.
One of the principal differences between domesticated dogs and wolves is that most dogs come into oestrus and breed twice a year. Only the most primitive breeds, including the Australian dingo and the Basenji (an African breed), resemble their wild ancestors in breeding only once a year. In a biological context, primitive just means coming first (as in ‘primary’). The word does not imply inferiority or simplicity.
Which special form of natural selection relating to reproduction could be eliminated entirely under domestication?
Humans determine which adults breed. Sexually receptive females may be confined with only one male, effectively eliminating sexual selection.
Which other natural evolutionary processes that determine genotypes as well as phenotypes would continue to operate under domestication?
Genetic drift and other forms of non-adaptive evolution.
Domesticated animals are usually held in small, isolated populations with limited opportunities for interbreeding. For centuries, drovers herded large numbers of non-breeding pigs, cattle, sheep, geese and poultry to market for slaughter, often over hundreds of kilometres. However, the breeding stocks were usually kept within a small area because moving sexually active bulls and large boars over long distances was much less practical.
Domestication is different from taming of wild animals. Certain individuals of many species, especially juveniles and females, can be tamed and live in captivity where they are protected from predators and provided with food and shelter. Domesticated livestock are also fed and protected, but many generations breed and spend their entire lives in close association with humans.
List the similarities and differences between evolution under domestication and natural evolution.
Selection operates, determining that not all individuals born reach maturity and breed successfully.
Selection is for certain characters or combinations of characters but it only causes evolutionary change if the phenotypic characters have a genetic basis.
Mutation and genetic drift can occur leading to adaptive and non-adaptive evolution.
Artificial selection as well as natural selection operate, but not necessarily in the same direction.
Artificial selection may favour characters that would have been eliminated under natural selection.
Sexual selection is partially or entirely eliminated.
Artificial selection can be severe, with only a small fraction of each generation becoming parents of the next.
Population size of domesticated livestock can be even smaller than natural populations, which promotes inbreeding.