Humans do not merely affect artificial selection; they are the cause of it. Artificial selection occurs when humans purposefully pair animals with desirable traits in order to obtain offspring with similar traits. Over time, the process can produce drastic changes in bred animals, such as those exhibited by the many different species of dogs, which have all resulted from the artificial selection of wolves.
Humans engage in the artificial selection of plants as well as animals. Kale, cabbage, broccoli and cauliflower, for example, have all been created from the mustard plant. By emphasizing some traits and de-emphasizing others, humans have created a number of different vegetables from the stock of a single, ancestral species.
Many different pets look and behave as they do because of artificial selection. Fish hobbyists, for instance, have produced an incredibly diverse array of goldfish and other popular species. Many amphibians and reptiles that are popular among hobbyists no longer resemble their wild counterparts.
Artificial selection contrasts with natural selection and sexual selection, which both occur outside of the influence of humans. Natural selection results from the selective forces acting on a species or habitat. Sexual selection results from the individual preferences of animals and the way they select and acquire breeding partners.