I've just been reading the chevrotain thread, and there's been a lot of discussion about different species, and also how hard it is to tell some of the different species apart. I am curious to know what makes an animal a different species, rather than a sub-species. Is it always defined the same for all family groups or does it vary? Why are sub-species chAnged to species and vice verse?
Simple answer is that there is no answer. The term species has no finite definition, or rather there are about 20 definitions for what a species is. Under some definitions an animal might either be a species or a sub-species or an eco-type or something else. It is the hardest concept to pin down in biology.
I agree, it´s very hard to find one specific definition, that would apply for all organisms. And you will find exceptions for every definition.. When I´m asked this question, I usually go with the interbreeding answer. Subspecies can interbreed and produce fertile offspring, species can´t. (Amur Tiger would breed with Bengal Tiger and produce healthy offspring, Lion wouldn´t breed with tiger naturally, therefore ligers are infertile hybrids.) But I´m sure you will find exception for this rule as well..
[ame=http://en.wikipedia.org/wiki/Species]Species - Wikipedia, the free encyclopedia[/ame] Good luck. Oh, and Stefka: Black-eared Miner, Black Stilt, Brown Teal. All that jazz. Generally I just use if two individuals breed and are affected by Haldane's rule, then they are two different species.
Stefka's example is known as the biologic species concept. This is the traditional one used by many older zoologists. Problem with this concept is that it produces a very gray area. The tiger x lion example is a classic case of the BSC. Both of these cats are fairly far apart from each other on the Panthera genus tree. That said, I have seen 2nd generation lion x tiger hybrids [(lion x tiger) x lion] so the hybridization rule is not absolutely 100%. Another case would be from the Berlin zoo where they accidentally crossed and Arabian Oryx with an Addax (two different genera!) and got a fertile offspring which was then bred again to another oryx (I've heard different reports if it was another Arabian or a Scimitar-Horned). The other widely used concept is the phylogenetic species concept (PSC). In this definition, species are distinct points along an evolutionary tree. All the members of these species have definable, inherited characteristics that separate them from other PSC species. This concept has far more distinct species, and for true purists of the PSC there are NO subspecies. Either it is a species or it isn't. What is to be taken away from this? The concept of a species is a man-made definition and nature doesn't play by our rules.
Well said. Speaking about the crossbreeding, how about the recent cases of Polar Bears breeding with Grizzly Bears? Do they produce fertile offspring?
Probably so. There are some who would argue that the Polar Bear isn't even a true species as it is so closely related to the Brown Bear (only about 10,000 years separate them if I remember right). The Polar Bear may just be a sub-species of the Brown Bear under the BSC. Only under the PSC would they be a species.
Bornean and Sumatran orangutans are nowadays regarded as full species, rather than subspecies as previously. They can hybridise and the offspring are fertile.
The blurry line of hybridization is why I personally prefer the PSC. Looking though bird lists for my recent forays into birding, I find that the PSC has really already been applied to the bird world. Birders are well known splitters. Look at all the different kinds of song birds. If these were large mammals we would have traditionally call all of these subspecies.
When I get home I'll check my books, but I remember reading that the average genetic difference between Mammals is something like three times that of Birds.
I saw this animal; it was in the (East) Berlin Tierpark. It was definitely a triple hybrid involving addax, Arabian oryx and scimitar-horned oryx.
Yes, there are well documented brown bear x polar bear hybrids that have themselves given birth when mated to either a brown bear or a polar bear.
I find it fascinating. I´m not a fan of crossbreeding, but when it happens naturally in the wild, it makes you wonder..
Groves and Meijaard, the authors of the two* recent chevrotain papers are strong proponents of the phylogenetic species concept. In both papers they specifically say they use psc. *I switched the years in my post in the chevrotain thread. The paper that resulted in splits in Tragulus is from 2004. 2005 is the paper for Moschiola splits. Hybrids are as problematic in psc as in bsc. Hybrids are perfectly fine under bsc, even hybrids that are fertile. Ernst Mayr, widely considered the father of bsc, frequently used the terminology "reproductive isolation" instead of the often quoted "if they can produce hybrids that are fertile, they're one species". There is a small but significant difference between the two: Reproductive isolation may occur even if two species are able to produce fertile hybrids. Carrion crow and hooded crow is a good example. Two decades ago, everybody believed they were subspecies of one species because they hybridize and the offspring is fertile. A wide range of studies have now confirmed assortative mating (one study had a different result but that is another discussion) and the hybrids, even if they can reproduce, are evidently not fully compatible since the hybrid zone remains quite narrow. If they had been fully compatible, one would have expected a broader hybrid zone in highly mobile species like a crow. A more accurate version of the commonly quoted sentence, more closely matching how taxonomists use the bsc today, would therefore be something like “If they can and will hybridise under free choice, and the offspring is fertile and without lowered fitness, they’re one species.” The lion X tiger and Arabian oryx X addax examples are only potentially valid if using the oversimplified version. They do not match the actual version by Mayr and alike. So what do bsc taxonomists do in two populations that have no contact in distribution? They compare differences (in genetics, morphology, voice, whatever) to related species that are in contact and are known to maintain their integrity as separate species. If the non-overlapping pair have differences that are similar to or exceed those of the pair that are in contact, it is reasonable to assume the non-overlapping also are separate species. If the differences are closer to populations that are in contact and do not maintain their integrity (=subspecies), it is reasonable to assume they also should be treated as subspecies in bsc. Parapatric populations that hybridize and clines are potentially more problematic in psc because it lacks specific requirements on border, beyond the fact that a species has to be monophyletic and differ from other mon. species. If I took a genetic sample of you and your blood relatives, I can document differences compared to your neighbour and his blood relatives. Separate species? Of course not and this is a faulty example, but I could claim that because of the lack of border requirements in psc. One noted mammal taxonomist typically uses cranial measurements and he has split numerous populations as psc species because of that. In several cases he has also documented differences in pelage but this is not sufficient for species status to him. That's only possible because of the inconsistent requirements in psc. Interestingly, using the standard definitions of psc, there are a few groups where one arguably could say there are more bsc species than psc. For example, cichlids of the Great African Lakes where some groups are essentially inseparable genetically and only maintain their differences when everything within their natural habitat is right. One could almost argue that they are morphs, except they segregate and behave as bsc species under natural conditions. That's why the Lake Victoria cichlid program has caused such problems to zoos. After a few generations, the captive cichlids often end up looking like the generalist species. Another example are the capuchinos group of seedeaters in South America that genetically appear identical (=one psc species) but they differ distinctly in appearance and song, and maintain their integrity despite overlapping ranges (=several bsc species). It should be noted that there are questionable claims in the capuchinos publication but the fact remains that these bsc species are unlikely to be monophyletic if split out. Unlike in psc, monophyly is not a strict requirement in bsc. The status of this miner as a species has not been entirely resolved, Christidis and Boles 2008. All three examples are also potentially problematic because of their rarity. Very low population hinders natural mate selection and also means that remaining populations are likely to have lowered survivability because of inbreeding. This means that lowered survivability in hybrids, if it is there, may not surpass the lowered survivability in the species itself. However the brown teal leads to a valid example where rarity cannot be the sole explanation: The number of duck and goose species that are able to hybridize is baffling. At least among some members of the mallard complex there are cases where lowered hybrid fitness has not been conclusively documented. The crazy breeding behavior of mallards that will try to mate with almost anything means that mate choice is no limit. There are few groups with such almost unlimited hybridization capabilities and they do cause a serious headache to bsc proponents. One could argue, as a minority do, that the entire mallard complex is one species. This only results in other taxonomic problems. The bsc remains by far the most widely used in ornithology. Major taxonomic lists (Clements, Howard and Moore, BOU, AOU, etc) all use the biological species concept and their treatment form the basis for most ornithological literature. IOC used a bsc baseline but some of their changes, especially in groups with little hard data where it eventually could go either way, have leaned towards "semi-psc". "Semi-" because they maintain subspecies which isn't possible in true psc. Either populations differ (separate psc species) or they don't (one psc species). There is no middle way. Either way, IOC is still largely a bsc list. Beyond very small groups there are very few bird lists where psc has even been attempted. Among the very few examples are the UK400 club (its taxonomy, based on the opinion of a single man, are sometimes questionable no matter what concept you follow) and the list to birds of Mexico by Navarro-Sigüenza and Peterson. Mammals remain overall mostly bsc but a some taxonomists have pushed strongly for psc in some groups, mainly primates and ungulates. This has resulted in a big shift. But these all use a peculiar "semi-psc" with subspecies. This is not a case of puritism, contrary to an earlier suggestion. Subspecies are simply not possible under the concept. Either two population differ (=species) or they don't (=not species). If peope want subspecies they'll have to use another concept than psc. Regardless it'll be a long time before any full psc mammal list has been made. Changing primate and ungulates to psc was relatively easy. Who's going to do the same with bats, rodents and shrews The most widely used for reptiles is bsc but especially turtles have moved towards psc with recent publications by IUCN Turtle Taxonomy Working Group and others. They've not gone full psc yet and I could be wrong in my prediction but their direction seems clear. Visitors to the Galápagos should remember this when they see the land tortoise on different islands. Fish and amphibians are also mostly bsc but with ongoing shift towards psc. However any classification of reptiles, amphibians, fish and arthopods is at best preliminary. Uncertainties of some basics in the classification of these groups combined with the huge number of underscribe species –thousands in the first three and likely millions in the last– makes any claim of an overall solid classification questionable. They make classification of birds and mammals seem straight forward. In groups where asexual reproduction is widespread, such as some plants, corals and bacteria, bsc is essentially useless. Psc also faces some problems here but it still has an indisputable advantage when dealing with asexual speciation. Whether this is the case or not, it doesn't really say much on speciation. There are huge variations depending on group, even depending on temperature or latitude (lots of publications, e.g. I, II and III). 1% difference may be enough for bsc speciation in one group but far too little in other groups. That's why we have to compare relatives and don't use completely random taxa as outgroups. That being said, I think most taxonomists believe that more splitting remains in mammals compared to birds, even if using the same species concept. There have always been more scientists working with bird taxonomy than mammal taxonomy so the former are ahead. All this babbling aside, the final sentence in post 5 by jbnbsn99 is easily the best way to summarize this entire thread.
I am not sure if someone already discussed this, but how do you define a sub-species as compared to natural variation within a species? For example, I was at a zoo the other day and saw two dromedary camels: one was very light in colour, and the other very dark surely natural variation rather than a camel sub-species?
With the camel you are looking at a domestic species not unlike dogs. We bred those traits into these animals. Within natural occurring species, color morphs like these are very rare, like a true white tiger. An actual white tiger hasn't been seen since the 1950's. A great example of a subspecies that holds up in both the BSC and PSC is the Common Zebra (Equus quagga). There is what is known as a cline from north to south. The southern animals have less stripes as in the extinct Quagga whereas the northern animals like the Grant's Zebra have full black and white stripes. There is little to no morphological differences between the southern and northern zebras.
Oh yeah! I forgot that they were domesticated! Regarding the zebras, are you saying that the Common Zebra is one species, but the stripe pattern varies depending on the region, but that they are still all just the one species?
