Speciation is the process by which new species are formed. This occurs when populations become separated from the rest of the species. At this point, the isolated group will respond independently to natural selection until the population becomes reproductively isolated. The group is then considered a new species. If a population becomes reproductively isolated, then individuals within the population will no longer exchange genetic material with the rest of the species. At that point environmental factors (for example, natural selection) will work on the genetic variation within that population until it has become a new species.