Scientists do not agree on exactly how new species form; they do not even agree on a single definition for the term “species.” Some suspect that new species form when a population of individuals becomes reproductively isolated from others of its species. Over time, mutations appear in the genetic code that permeate the isolated population, causing a new species to arise.
Other scientists propose that species may arise when a population within a species begins changing enough that it fails to compete or cooperate with the original population. At some point in time, the changed population becomes a new species. This often occurs as species change their morphology and behaviors.
Another factor that may cause species to form is sexual selection. A subset of organisms within a population may begin finding different traits attractive or may adopt new courting behaviors that are not effective with the species at large.
Artificial selection, which occurs when humans selectively breed organisms to enhance desirable traits, likely causes new species to appear. However, it is not always clear when a new species has arisen by this technique. For example, poodles and German shepherds are the same species, but they are clearly different varieties of the animal.