Selective breeding

Last time

Before we start: last time, in Natural selection, resistance and selective breeding, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.

Humans choose the parents

Every cow on a dairy farm started as a wild animal that gave only enough milk for its calf. Farmers kept choosing the best milkers as parents, generation after generation. That is selective breeding, also called artificial selection. Today: the steps, the examples, and the problems it can cause.

What it is

Selective breeding means humans choose which individuals breed, to produce offspring with useful features. It is also called artificial selection, because people, not the environment, do the selecting. It only works if there is genetic variation, and if the chosen features are inherited.

The steps

Step one: choose parents with the feature you want, such as high milk yield or disease resistance. Step two: breed them together, and step three: from the offspring, select the ones with the best feature. Step four: repeat for many generations. The feature becomes stronger each time.

Examples

Cattle are bred for more milk or more beef. Chickens are bred for more eggs or faster growth. Wheat is bred for disease resistance, a higher yield, and short, strong stems that do not blow over. Broccoli and cauliflower both came from the same wild plant, by choosing different features.

Same as natural selection?

Both need genetic variation, and both need reproduction, so that alleles pass to the next generation. Natural selection has competition for resources and the environment selecting. In selective breeding, a human selects. Selective breeding is much faster, because the human picks the parents every time.

Problems

Only a few individuals breed, so genetic variation in the population falls. Closely related animals may breed together, which increases inherited diseases and reduces fertility. If a new disease arrives, all the near-identical organisms may be affected, since none are resistant.

Why farmers do it

Higher yields mean more food from the same land, and animals that grow quickly save feed and money. Disease-resistant crops need fewer chemicals, and crops that ripen together are easier to harvest. These benefits are why most food crops and farm animals today are selectively bred.

Pedigree animals

Pedigree dogs are bred for looks. Some breeds have flat faces or very large heads, which can cause breathing or birthing problems. A small number of parents means shared alleles, so harmful recessive alleles appear more often. Breeders can reduce the risk by checking family trees and avoiding closely related parents.