Obviously ducks and geese and swans look like each other. And housecats and tigers ditto
But what about Panda’s and raccoons or hydrox(sp? maybe that is a cookie? 
but you know that little rodentlike animal) that is related to an elephant.
What criteria do they use?
Also that lizard like taratula(sp?) that New Zealand critter that looks like a lizard but isn’t. Why isn’t it?
They used to be (from the time of Linnaeus to probably around the late 60s and 70s) based on morphological similarity (how much their organs and body parts were similar) down to the level of cellular structure. Now they’re based on DNA similarities, which, in most cases, followed the morphological method fairly well. Some animals got switched to different genera and even families, but not many, considering the total number of known species.
Plants, on the other hand, are still being shuffled around on the taxonomic tote board.
DNA analysis tends to focus on rRNA (ribosomal RNA) sequences, since they are so “basic” to life and remain so “constant”.
This sort of finicky anatomical detail is often what jayjay was referring to when he mentioned “morphological similarity”; skull bones seem to be used a lot for this sort of stuff. Basically, these fine details of anatomy indicate that tuataras split off from lizards hundreds of millions of years ago.
A widely-used method of sorting related taxa is known as Cladistics.
If you have Acrobat Reader, here is a PDF file, http://www.gwu.edu/~clade/faculty/lipscomb/Cladistics.pdf, called Basics of Cladistic Analysis.
If you don’t have Acrobat Reader, get it! It can be obtained here. A LOT of interesting info is available in this format.
Bill
NOTE I could not get the first link to display in the normal manner, so I had to use the ‘sloppy’ method of showing it. Sorry.
Skulls are used because they have lots of little bones (and soft features) that vary from species to species. Among closely-related species, a leg is a leg is a leg, for instance. But But the skull, with so many different parts that could potentially change, is more likely to show some difference. It also houses feeding structures and sensory organs, which are particularly sensitive to evolutionary pressures.
Pandas, bears and racoons were separated out based on protein chemistry in the late Eighties before the heyday of DNA analysis.
I understand that Pandas aren’t Raccoons anymore because they’re Bears, but are Raccoons now grouped with bears too?
…And Ginger isn’t Ginger anymore because she’s Mary Ann.
Panda’s aren’t bears. Bears and Panda’s diverged relatively recently from a common ancestor (15-20 million ya). The racoon/red panda line diverged somewhat earlier from the same evolutionary line (35-40 millio YA). Both lines split from the ‘main’ carnivore line about 50 millio YA. So raccons are part of the bears lineage, but neither they nor Pandas are classified as bears.
Taxonomy consists of man-made classifications, based on morphological and other features noted in the postings. However, Nature does not consist of such nicely distinguished genera. Note the platypus, which we classify as a mammal because it has mammary glands; however, it lays eggs. There are other animals which do not fit nicely into our categories.
While I sincerely hate to disagree with you, since it really messes up at least one former pride of my little-known trivia collection, http://www.infobiogen.fr/people/dessen/prp/taxo_theria6.html lists Ailuropoda melanoleuca under the Ursidae, and so do at least two others that I’ve looked up. I was shocked, too.
Ailurus fulgens, the lesser or red panda, is still listed under Procyonidae, though.
OK, I’ll confess that the reference I’m using is about 10 years old. Any idea what they decided to base the re-classification on?
barbitu8:
Actually, the twin-nested hierarchyis remarkably consistent. This makes it easy to classify things (and also is great evidence for common descent). Things would be really tricky if we had birds with mammary glands or fish with cusped teeth, but we do not.
There is nothing wrong with an egg-laying mammal. Mammals can be divided into three suborders: placentals, marsupials, and monotremes. Placentals dominate, causing many people to confuse their characteristics with those of all mammals. It would be a problem if we had feathered mammals, but egg-layers fit into the taxonomical picture just fine.
Gaspode:
I don’t know. I couldn’t find an online source that went into detail about why it was changed. I think, as someone mentioned earlier in this thread, that it was a result of the protein comparisons done in the late 80s. Considering that my last biology class was in Spring 1990 and I’m no biologist, it’s no real surprise that I hadn’t heard of it before now. Like I said, it shocked the hell out of me. I’ve been bringing up the “Actually, pandas aren’t really bears” (now pseudo-)factoid for years…
And, as an aside, I just think it’s really neat that the binomial for the panda translates to “Black-and-white catfoot”.
Perhaps he thought I meant the lesser panda. I should have
made myself clearer.
This annotated classification of Carnivora says that the grouping of Ursidae is based on a tendency toward omnivorism, skull morphology, and dentition (thought Britannica’s individual article on the giant panda says that its dentition is somewhat different from other bears, and some taxonomists suggest placing it in its own family Ailuropodidae.
I’m not a professional zoologist by any means, but at least
superficially, giant pandas seem to be extremely bearlike.
So how does that hydrox or whatever that little rodentlike animal is link to the elephant.
Or why isn’t the rocky mountain goat a goat? It looks like one
I guess how many differences does it take to make a new species?
For instance rabbits aren’t rodents but look like they would fit in the family. But what if they only had one different trait from a rodent. Why couldn’t they be a rodent with a split lip?
Hyrax. Hydrox really is the pseudo-Oreo.
But Hyrax isn’t a member of Proboscidae, and the Berkeley website (http://www.ucmp.berkeley.edu/mammal/mesaxonia/mesaxonia.html) says that there is a strong faction for placing it closer to the odd-toed hoofed mammals (Perissodactylae).
But I’ve been wrong before (most recently about six or seven posts up this thread), so take me with a grain of salt.
Not quite true. Taxonomy is possible because related species share common ancestors. Our common ancestor with the platypus laid eggs too. There were all kinds of mammal-like reptiles back in the Permian until they went extinct to make room for the small but clever dinosaurs.
Anyway, common descent is recongized as the fundamental principle of taxonomy. Two species that at one time were one species are deemed related. The farther back the common ancestor, they farther back the taxonomic level they share.
So, coyotes and wolves are very similar, and were effectively one species very recently. So, they share the same genus. Coyotes and foxes are less similar because they were the same species much longer ago, and so are only in the same family. Coyotes and bears are even less similar because their common ancestor was tens of millions of years ago, so they are only in the same order. Coyotes and capybaras have common ancestor even farther back, probably more than 65 million years ago, so they only share the same class, mammalia. Coyotes and alligators share a common ancestor back in the paleozoic, hundreds of millions of years ago, so they are grouped in the same phylum of vertebrates.
So, taxonomy does (or should) represent an actual physical reality, that of common descent with modification.
