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.
