Selection
Humans have been selecting features in animals and plants they find desirable for thousands of years. The animals and plants we find in farms today are a good example.
Wild wheat has very small seeds but has big ears. Primitive farmers grew this wheat for its seed so they could make flour. They kept some seed to plant for the next year. By keeping the bigger seeds for replanting, the plants which grew in the next year produced bigger seeds. This made them easier to harvest as well as provide more food. If this trend of keeping large seeds for replanting continues over generations, the wheat grow would produce large seeds.
This is known as artificial selection, as the characteristics which are favoured are selected by humans. Artificial selection involves preventing animals and plants which have undesirable qualities from breeding while allowing those with desirable ones to breed. This is why it is often known as selective breeding.
Dogs are all descended from wolves. Characteristics which were found desirable were breed with others. For example, if a smaller animal was needed then two small wolves were breed together. Over many generations the characteristics become so strong that new breeds and varieties are produced.
Selective breeding can cause problems. In the selection process inbreeding is common. This means that harmful ones can be passed on too leading to defects which may occur after many generations. Also, any weaknesses they have will be passed on too. An example is the potato famine in Ireland. All the potato plant were dervived from one original parent. A disease called blight affected the potatoes rendering them inedible. As all the plants were genetically identical, all the plants got the disease and a famine ensued.
