How is artificial selection different from genetic engineering?
Both genetic engineering and artificial selection allow humans to change a species so that its members are better suited for human needs. However, the mechanism for change is different. Artificial selection selects for traits already present in a species, whereas genetic engineering creates new traits.
In artificial selection, scientists breed only individuals that have desirable traits. For example, scientists may breed (cross pollinate, in this case) only the highest yielding crops with one another for many generations. The result is a population of plants that all produce a higher yield (e.g., abundant fruit production) than other members of the species. Through selective breeding, scientists are able to change the traits in the population. Evolution has occurred.
In genetic engineering, scientists use tools of DNA technology to directly manipulate a genome. One way to change the genome is to insert genes from other organisms. For example, some cotton plants have a gene encoding a bacterial toxin in their genome. The cotton plant therefore becomes toxic to pests like moth caterpillars that typically eat the plant's leaves. Toxic cotton plants are different from their un-engineered ancestors. Evolution has occurred.