Artificial selection

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  • Created by: portia
  • Created on: 02-08-17 19:54

When humans purposefully apply selection pressures to populations, the process is known as artificial selection

The deveopment of modern breeds of cattle; for many years people have tried to 'improve' their cattle. Desired features include docility (making the animals easier to control, fast growth rates and high milk yeilds.

  • Increases in these characteristics have been achieved by selective breeding
  • Individuals showing one or more of these desired features to a larger degree than other individuals are chosen for breeding
  • Some of the alleles conffering these features are passed on to the offspring and the 'best' animals from this generation are chosen for breeding again
  • Over many generations, alleles conferring the desired characteristics increase in frequency, while those conferring undesired characteristics by the breeder decrease in frequency
  • In many cases, such 'disadvantageous' alleles are lost entirely

Such selective breeding of dairy cattle presents the breeder with problems

  • The animals are large and take time to reach maturity
  • The gestation period is long and the number of offspring produced is small
  • A bull cannot be assesed for milk production since this is a sex-limited trait (not sex linked)
  • Instead, the performance of the bull's female offspring is looked at to see whether or not to use the bull in further crosses
  • This is called progeny testing and is a measure of the bull's value to the breeder

It is important to realise that selective breeders have to consider the whole genotype of an organism, not just the genes affecting the desired trait. Within each organism's genotype are all the alleles that adapt to its particular environment. These genes are called background genes:

  • suppose that the chosen parents come from the same environment and are from varieties that have already undergone some artificial selection. It is likely that such parents share a large number of alleles of background genes, so the offspring will be adapted for the same environment
  • but suppose instead that one of the chosen parents comes from a different part of the world. The offspring will inherit appropriate alleles from only one parent. It may show the trait for being selected for, but it may not be well-adapted to its environment

Crop improvement

The same problem is seen when a cross is made between a cultivated plant and a related wild species.

  • Although most species will not breed with different species, some can be interbred to give fertile offspring. Such species are usually those that don't normally come into contact with one another, because they live in different habitats or areas
  • The wild parent will have alleles that are jot wanted and have probably been selected out of the cultivated parent

Farmers have been growing cereal crops for thousands of years and although these early farmers knew nothing of genes and inheritance, they did realise that characteristics were passed on from parents…

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