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RE: Polyphyletic

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Posted by: CKing at Thu Oct 2 08:55:53 2008  [ Report Abuse ] [ Email Message ] [ Show All Posts by CKing ]  
   

>>Frogs and birds share thousands of traits that derive from common ancestry... not convergence. They both have limbs, for example.>>

Yes they both have limbs, but that begs many questions. On what basis does "limbs" allow birds and frogs to form a monophyletic or paraphyletic group? After all, lots of animals have limbs. For example, insects (e.g. butterflies), molluscs (e.g. octopus), arachnids (e.g. spiders), crustaceans (e.g. crabs) and of course many tetrapods (e.g. alligators) have limbs. So, why use "limbs" to separate frogs and birds from all these other animals? How does limbs exclude these other animals from this group? It doesn't. In fact, limbs is a convergent character between molluscs and tetrapods. Therefore, a group consisting of animals with limbs would be polyphyletic because "limbs" is a convergent character. I know of no practicing systematist who would consider "limbs" a valid taxonomic character. However, some natural groups can be identified by the number of limbs that they possess. Spiders have 8. Insects have 6, and of course tetrapods have 4. But even eight limbs is not itself a good taxonomic character by itself, because octopuses also have 8 limbs.

So, unfortunately, you have not provided a single synapomorphy that could be used to unite frogs and birds as either a monophyletic or a paraphyletic group. If I were a professor, I would give you an F. You flunked. LOL.


   

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