Selective Breeding

Selective breeding (also called artificial selection) is the process by which humans deliberately breed organisms with desirable characteristics together, over many generations, to produce offspring with those characteristics consistently expressed

Humans have been selectively breeding plants and animals for at least 10 000 years, ever since the first farmers chose which crops to plant from this year's harvest and which animals to keep for next year's breeding. The result is the modern range of crops and farm animals: large, productive, fast-growing, disease-resistant, and almost completely different from their wild ancestors.

The four-step process

The basic procedure is the same for any species:

1. Identify the desirable characteristic you want. This might be high milk yield, large fruit, disease resistance, fast growth, or a specific colour.
2. Choose parents from the population that show the characteristic most strongly. Breed them together.
3. Select the offspring that also show the characteristic. The rest are discarded (not allowed to reproduce).
4. Repeat for many generations. Each generation, only the best examples are kept and bred together. The characteristic becomes more and more pronounced in the population.

After enough generations (often tens to hundreds), a new variety has been produced in which every offspring reliably shows the chosen characteristic. This is called a breed in animals, or a variety or cultivar in plants.

Exam note: an answer that only mentions one round of breeding (choose two parents → breed them together → done) is incomplete. The process must be repeated over many generations for offspring to reliably show the desired trait.