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Re: CONVERGENCE HAPPENS
Larry Febo wrote:
>Seems to me that we had better come up with definative answers as to what
>exactly constitutes "Convergence", so that it`s not used as a "catchbasin"
>for whatever we cannot fit into our Phylogenetic sequences!
Well spoken. Phylogenetic systematics regards "convergence" as just
one type of homoplaisy; two organisms share the same character state, but
did not inherit it from a common ancestor. In the case of "convergence", I
suppose the character would be derived (at least locally on the tree) rather
than ancestral (in which case it would be a "reversal", unless both animals
had independantly reverted to the ancestral condition, in which case it is
"convergence" again... see why just saying homoplaisy makes it easier?).
This does not distinguish convergence from parallelism, but then I've never
really heard a clear, objective distinction between the two* (is parallelism
convergence below the "family level"?).
* BTW: to everyone who loves to put their $0.02 on this sort of thing, I do
actually know the supposed difference between the two "phenomena" of
convergence and parallelism. I'm wondering if there is a non-normative
approach to differentiating them.
>...sometimes convergence happens!....(SOMETIMES it is used when there is no
>other way to explain such similarities to ones liking).
Actually, homoplaisy seems to happen a lot! If there were no
homoplaisy, phylogenetic reconstruction would simply be a matter of
idnetifying and ordering synapomorphies. All of life would fit into a neat
heirarchy of accumulating shared derived characteristics. But it ain't that
kind of party. Homoplaisy is, depending on what group you look at, anywhere
from just frequent enough to make phylogenetic reconstruction require more
than a scrap of paper to work out to downright rampant (e.g. theropod
>Due to embryological evidence that seemingly directly
>contradicts a Dino-Bird connection, he automatically dismisses any (and all)
>Dromaeosaur-Bird similarities as Convergence!
He is (IMHO unjustifiably) overweighting one piece of evidence over
a lot of other evidence. Just because he does this doesn't mean others every
case of cited homoplaisy is similar. If a cladistic analysis gives you a
certain result, you can (generally) rest assured that that result has the
*minimum* possible number of assumed homoplaisies. In fact, that's what
parsimony (the principle upon which cladistics programs operate) is for:
making the best hypothethetical tree possible by minimizing assumed
homoplaisies. So do not assume that Matt is lightly dismissing
bird/pterosaur similarities as "convergence". His statements to this effect
are influenced by studies which suggest that grouping birds and pterosaurs
together results in *MORE* "convergence."
>I just wish there were a way to prove or disprove convergence. In all cases
>where it is used, the claim for its existance should REQUIRE proof!
And that's *exactly* what cladistics is for.
Matt Troutman wrote:
>Gaviiforms, podicepiforms and hesperornithids share
>many features realted to swimming, and some have argued for monophyly of
>these three groups, but in retrospect these features are probably not
>homologous because of the subtle differences in knee structure,
Ah, but recall that evolution happens, and subtle differences in
knee structure are the sort of thing evolution produces, and evolution does
not care whether it starts from one knee structure (say, the one you don't
think is homologous) or another. So, just because things are built
differently doesn't mean they weren't inherited from a common ancestor. I'll
give you an example... my arm is built very differently from an ichthyosaur
paddle... care to guess if they're homologous?
Ditto. I don't swim like an ichthyosaur, neither do dolphins.
>and shared derived features with other birds.
And that, friends and neighbors, is where you find your evidence...
do each (well, 2 out of 3) of these groups share more characters with
*other*, non-swimming birds than with their aquatic pals? This is where
you'll find your convergence, Larry. This is where you find your evidence,
Matt (although the anatomical and behavioral stuff is good data).
>If we try to account for all convergence that happens in all our accepted
>phylogenies, we end up with birds and mammals as sister-groups, [...]
Actually, no. If we try to account for all convergences, we get a
phylogeny similar to what we have now, since that's what cladistics is
supposed to do. If we were somehow able to eliminate all homoplaisy, well,
that would be a trick.
Jonathan R. Wagner, Dept. of Geosciences, TTU, Lubbock, TX 79409-1053
"Only those whose life is short can truly believe that love is forever"-Lorien