Humans have had a profound influence on their environment and the species with which they share it. One of the major steps toward our development of civilizations was the domestication of plant and animal species for food and labor. We were able to take wild grasses and grains and selectively breed them to produce greater yields. We bred cattle, sheep and horses to be larger, stronger and more docile. We even bred wolves to help us maintain and defend these other animals.
The process of domestication is called artificial selection. Like natural selection, artificial selection acts by allowing differential reproductive success to individuals with different genetically determined traits in order to increase the frequency of desirable traits in the population. However, unlike naturally selected traits, artificially selected traits do not necessarily convey greater fitness. Instead, artificially selected traits are based on what the person breeding the plants and animals desires. These traits, which can range from longer cobs in corn plants to a particular coat color in dogs, are selected for by allowing only individuals that possess the trait to reproduce, while those that lack the trait are prevented from reproducing.
Because it lacks the control of fitness needing to increase fitness, artificial selection can cause problem traits to predominate in a species. For example, Dobermans are a breed of dog that has been bred for a certain appearance. In the process of selecting for this appearance, a genetic defect has increased in frequency in the population. This defect causes narcolepsy, a condition that causes these dogs to uncontrollably fall into deep sleep. Clearly, this condition does not increase the animal's fitness and would be strongly selected against by natural selection. However, since these animals have been subjected to artificial rather than natural selection for generations, the defect has spread in the population.