9.4 - Selective Breeding

What selective breeding is

• Selective breeding (also called artificial selection) is when humans choose which plants or animals get to breed, so the genes for a chosen characteristic are kept and made more common in the population.
• People have done it for thousands of years - it's how edible crops were developed from wild plants and how animals such as cattle and dogs were domesticated.

Organisms are selectively bred for characteristics that are useful or attractive, for example:

• Crops that resist disease, or that give a higher yield of grain, fruit or other food
• Animals that produce more meat or milk
• Animals with other useful qualities, such as a gentle temperament in dogs
• Decorative plants with large or unusual flowers

Combining two characteristics

• Two characteristics can be combined in one variety by crossing, then selecting the best offspring over generations.
• Example - wheat:
  • Tall wheat gives a good grain yield but is easily flattened by wind and rain
  • Dwarf wheat resists the weather but yields less
  • Crossing the two and selecting over generations produces a dwarf, weather-resistant wheat with a high yield

The process of selective breeding

• Selective breeding means choosing which organisms breed together, so that offspring gradually develop a desired characteristic.
• The process is the same whatever the characteristic, and is repeated over many generations.

The steps

1. From a mixed population, choose the individuals that show the desired characteristic most strongly - for example, the cows and bulls that are largest and best for producing meat.
2. Breed these chosen individuals together.
3. From their offspring, choose the ones that again show the desired characteristic best, and breed those together.
4. Repeat the selecting-and-breeding over several generations.

Each generation, the characteristic becomes stronger and more common, until eventually all the offspring show it - after several generations the whole herd has a very high meat yield.

Natural and artificial selection compared

• Both change a population over many generations by deciding which individuals reproduce.
• They differ in who or what does the choosing and in their effects on variation.

Who does the selecting

• Natural selection - The environment does the selecting. The individuals that survive and reproduce are those whose characteristics happen to suit their surroundings, so it affects all wild organisms.
• Artificial selection (selective breeding) - Humans do the selecting. The individuals that reproduce are those with characteristics people find useful or desirable, so it happens only to the organisms humans choose to breed.

Speed and effect on variation