Splitting lumps, lumping splits, and other taxonomic shenanigans

18 07 2007

This past March I blogged about a Nature article in the special “Linnaeus issue” involving the problem with determining what actually constitutes a species. In fact, the very same day I also mentioned that a new species of Clouded Leopard (Neofelis diardi)was recognized in Indonesia, on which Darren of Tetrapod Zoology provided a much more thorough history. Now, as Rich at evolgen points out, a new PLoS editorial is suggesting that we take greater care in name (or renaming as the case may be) mammalian species.

Bornean clouded leopard (Neofelis diardi)
Kalimantan (Indonesian Borneo), Indonesia.
CREDIT: (c) WWF-Canon / Alain COMPOST
IMAGE No.: 112939

The authors of the opinion piece, Meiri and Mace, criticize biologists who split species because two or more populations of one species simply do not overlap (either caused by some barrier separating them over a distance or ranges abutting without overlapping). Regarding the caramel-colored clouded leopard, they write;

In resurrecting N. diardi, Kitchener et al. are relying on the phylogenetic species concept whereby species are defined as groups that share at least one uniquely derived character. They distinguish two clouded leopard “species” solely on the basis of pelage (fur color and pattern) characteristics, despite the fact that differences in hair color often reflect minor geographical varieties in many mammals. Borneo and the Malay Peninsula differ in several biotic and abiotic factors. Thus genetic and morphological differences between populations of the 144 mammalian species they share are to be expected, and there could potentially be equivalent evidence to merit specific status for all of these; an outcome that would certainly be unjustified.

Indeed, populations of one species existing in relative isolation from each other is not enough to gain a new species status (if that were true, we’d have many more species of White-Tailed Deer and Cougars throughout the Americas at the moment), but such differences do lead to allopatric speciation events, regional differences showing up over time as local ecologies become more distinct from each other. Such considerations, however, would essentially negate designation of new species by skin/feather/hair color alone, being that the colors or patterns of a particular species can vary without there being any other distinctive features (while perhaps still meeting the requirement of one “derived” character).

Species designation is not all about being objective, however, the designations of species and subspecies inflating or deflating the raw numbers of local biodiversity and having an impact on endangered species status. If, for example, an island is deemed not to have just one large population of a particular species but two populations of two slightly distinct species, then the overall numbers for these organisms fall and may incite groups to try and protect them. A subspecies or local variation is far less likely to gain protection than a distinct species, so splitting is a way in which some people could possibly ramp up the overall biodiversity count (therefore reducing the number of individuals in populations), in turn receiving more aid than others. I have heard this charge made many times, although I have not seen much actual evidence that this is the case (see this opinion piece from the April 2004 edition of Current Science).

So what about the renamed Clouded Leopard Neofelis diardi? What differentiates it from the species to which it previously belonged, Neofelis nebulosa? Fortunately, I was able to access the article “Geographical Variation in the Clouded Leopard, Neofelis nebulosa, Reveals Two Species” (available here)to have a look at the data myself. The researchers, using skins primarily from museums (2 being from private collections) comparing the pelage patterns of 57 clouded leopards (skulls were typically not associated with the skins, if available at all) in order to determine if there was some sort of geographical difference. The primary differences the researchers observed were described this way;

In summary, clouded leopards with large clouds (Neofelis nebulosa) tend to have fewer, often faint, spots within the cloud markings, and they are lighter in color, with a tendency toward tawny-colored fur and a partial double dorsal stripe, whereas clouded leopards with small clouds (Neofelis diardi) tend to have many distinct spots within the cloud markings, greyer fur, and a double dorsal stripe.

The reason for the striking difference in coat pattern may be due, the researchers hypothesize, to the Isthmus of Kra acting as a biogeographical barrier, essentially isolating the two groups of clouded leopards from each other. The differences don’t merely reflect inconsequential differences in variation (i.e. the authors of the PloS cite Merriam going overboard splitting bear species based upon the smallest differences), but rather a split that has caused a good amount of differentiation in the appearance of the Indonesian leopards. Hair color is not the only evidence backing up the split, however, mitochondrial DNA analysis (the research appearing in the same issue of Current Biology as the pelage paper) showing that the greatest genetic difference between the clouded leopards is that between the “mainland” species & subspecies and Neofelis diardi. What’s more, the difference between the two different clouded leopard species appears to be equal to or greater than the differences between other cat species, further supporting the differentiation of these leopards. The authors do admit, however, that their sample size was rather small and more study needs to be done, but the results we have as of now strongly suggest that there are two separate species of clouded leopard living in southeast Asia.

While Neofelis diardi is not yet officially recognized as a species (at least to my knowledge), I do side with those who regard it as a distinct species. Suggesting that the authors of the pelage study merely looked at a few coats and said “Gee, these look different” and attempted to create a new species is a bit disingenuous, and the current PLoS editorial does not address the genetic studies of Buckley-Beason, et al. other than to say they were carried out. Their point about taking great care to accurately distinguish species is well-taken, but they did little more than make a caricature of the Neofelis diardi studies, and they probably could have picked a better example (although the overall attention the clouded leopard story got would make it an obvious choice because of its visibility).

Given the complexity and diversity of species concepts, there will likely always be disagreements and arguments. Indeed, if we look at the history of life on earth, it’s probably a good thing the fossil record is fragmentary; if we had all the organisms that ever lived and could line them up according to direct ancestor/descendant relationships from the first life form to us, we’d probably have a hell of a time determining when to make a species division. In essence, we have given ourselves a designation of Homo sapiens, but millions of years ago, billions of years ago, our direct ancestors looked quite different than us and would receive very different names. Indeed, as much as taxonomy is helpful, sometimes I wonder if designations of extant species sometimes conflates the evolution issue, suggesting that “species” are end-products of evolution that do not continue to evolve (and if we have no close ancestors, that they came out of nowhere). How this could be helped/fixed, I do not presently know, at least outside of serious contemplation and study of the history of life on earth.



7 responses

18 07 2007
Zach Miller

And, of course, taxonomy is even murkier in the fossil record. The most minor skeletal differences between animals often, even today, receives a new species designation. Look at Microraptor zhaoianus, Microraptor gui, and Cryptovolans pauli. These are all probably the exact same animal (they’re all from the same place and time), and M. zhaoianus has priority.

The old breeding adage of speciazation doesn’t work anymore because, frankly, we can’t get every kind of animal in a room and see if their baby is fertile. Morphology will have to be the bottom line in a practical world, but since morphology is often in the eye of the beholder, there will always be arguments over it.

Heh…and to think, taxonomy is the subject I blog most about!

18 07 2007

Thanks, as always, for the comment Zach. Especially when it comes to Cope and Marsh, I’ve always been amazed at the amount of names different species end up racking up (especially due to juveniles, sexual dimorphism, etc.). Even Archaeopteryx had a whole slew of synonyms (it seems like everyone just called it something else at one time or another), and don’t even get me started on the whole Deinonychus=Velociraptor that came of Michael Crichton reading Predatory Dinosaurs of the World in doing background research for Jurassic Park…

I’m partial to morphology myself, but I know that just about every scientific discipline is going to have their own pet hypothesis about what works best, so I don’t see things becoming resolved anytime soon.

18 07 2007
Zach Miller

For the record, Paul’s work is generally quite good. He actually has a problem lumping too many animals under one taxon. You bring up an interesting point with Archaeopteryx–even today, depending on who you ask, there are up to three “valid” species of the little bird, even though they’re all from the same place and time, and there are only ten specimens. 🙂

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