A subspecies is defined as a geographic variant of a species. A subspecies is not necessarily an incipient species, even if some scientists seem to treat them as such. Some species may for example evolve sympatrically (in the same area) as its ancestor, and while it evolved it cannot be classified as a subspecies...
Best answer: A subspecies is defined as a geographic variant of a species. A subspecies is not necessarily an incipient species, even if some scientists seem to treat them as such. Some species may for example evolve sympatrically (in the same area) as its ancestor, and while it evolved it cannot be classified as a subspecies since it is not a geographical variant. Some populations that eventually become a new species may also not be different enough to be classified as a subspecies.
In many cases, a population becomes adapted to the local environment and it evolve differences that make it classifiable as a subspecies. However, sometimes a subspecies may be the result of neutral genetic drift and the differences are not adaptive even if it is noticeable enough to make it a different subspecies. For example a population of snakes has a neck ring that is interrupted in the middle, but the other populations have a complete neck ring. It is extremely doubtful that such a trivial character is an adaptation to the environment and therefore it may not be evolving into a different species, but it is nevertheless classifiable as a subspecies. If the adaptation to the local environment is drastic enough then a population may become so different ecologically that if it interbreeds with other populations, the hybrids would not be able to compete with one or both parental species. If so, then the hybrids will not be able to survive and the 2 populations may avoid interbreeding in the future due to natural selection against hybridization. If and when that happens, then a new species has evolved. Reproductive isolation, therefore, is how we can tell if a subspecies or a population (that was not classifiable as a distinct subspecies) has become a new species.
If a new species has evolved from an isolated population, then if and when it comes into contact with the old species again (secondary contact) they may not recognize each other as different (because they still have the same mating call, mating dance, phermone, or color pattern for example) and therefore they may interbreed at first. Because the hybrids are not as fit as either parental species, the two may then evolve premating reproduction isolation (by evolving different mating calls, dances, or color patterns or phermones for example). Their subsequent unwillingness to interbreed will therefore be good evidence that they are different species. Unfortunately, sometimes two populations that are adapted to different local environments may not come into secondary contact, and in those cases scientists would have to determine whether these 2 are different species or 2 different subspecies by comparing them. For example, the bonobo is geographically isolated from the chimpanzee. For a long time, scientists classified them as different subspecies of the same species. More recently scientists have determined that they are so different that if they meet, they may not interbreed, since the hybrids would be unlikely to fit into the lifestyles of either species. That is why most scientists now classify them as different species instead. It would be easier if these two were to come into contact in nature and inform us through their lack of interbreeding that they are indeed different species. But since they do not come into contact, scientists would have to determine their species status based on how different they are and whether they are different enough to refuse to evolve premating reproductive isolation should they meet in the future. Such determination is unavoidably subjective, but there is no objective means of determining species status in such cases.
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