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the tonight show (my final post on this)




Norm,
I just took the "halfway house" comment by Ron and tried to adapt it. I agree that it was somewhat confusing. I was sort of envisioning a cladogram as a street with driveways branching off to different halfway houses along the way. Sorry about that.
A simple Interstate analogy for cladograms might make this clearer. Cladograms are usually constructed in such a way that the highest evolutionary rates generally occur on the main line (Interstate), but once a group branchs off (forming an off-ramp or exit) their evolutionary speeds usually decline in comparison. They find their niche and often don't need to evolve as fast any more. Groups that have been going down their particular offramps extremely slowly for a very long time, we often call them living fossils (coelacanths, horseshoe crabs, etc.).
Once the dromaeosaurs branched off the Interstate (in the Jurassic?), they might not have slowed much (but all roads connected to that offramp eventually came to deadends---many marked with a K-T Extinction Sign). The next major offramp is probably Archaeopterygiformes (very short, probably never even got beyond end of Jurassic). Some people think the Alvarezsaurids are the next exit (while others think they branched off before the dromaeosaur offramp), but pretty much everyone agrees that the Enantiornithine offramp comes next (and their roads all dead-end by K-T as well). Then comes the Hesperonithiformes exit, and then the Ichthyornithiformes exit (both deadends by K-T). If the end-Cretaceous extinction had not occurred, many of these offramps would have eventually formed highways in their own right (as Tom noted in his own way), but it did occur and those groups dead-ended for good. That's life.
Then comes the exit for Palaeognatha Road--- they find their niches and slow down evolutionarily, and some were lucky enough that they haven't reached deadends yet (tinamous and the more derived ratites). After that last exit, some call it the Neognatha Interstate, the biggest part of the overall Aves Interstate System. Traditionally the Aves System has been regarded as a separate system in its own right, but phylogeneticists regard it as part of the increasing larger Dinosauria and Reptilia Systems. They usually name a new subsidiary system at each exit----which creates a lot of names----but they claim this is less arbitrary (and supposedly useful).
Anyway, the Neognatha System hasn't been mapped in detail yet, and the only thing most agree upon is that the first big exit is to the Galliformes-Anseriformes clade. The ordering of the rest of the exits have not been mapped out enough to form a definite consensus. The Interstate probably splits into various major highways, each with their own offramps. Which highway to call the Interstate then become rather arbitrary, but Passeriformes is the biggest and gets the most attention.
The major point is that the highest evolutionary rates (driving speeds) are usually on the Interstate, and things tend to slow down on most offramps (not always though). Therefore in evolutionary distance, most of the present neognath groups are a lot farther away from the dromaeosaur exit than the present paleognaths are. The paleognaths slowed down a long time ago, while the neognath evolutionary radiation continued to race ahead. By comparison, the crows and jays (in a long line of evolutionary speedsters) are much, much further down the Interstate System. Just how far depends on which part of the genome you look at, so there is no single answer to that question. But the overall evolutionary distance from dromaeosaur exit to any present paleognath is probably shorter than to any present neognath (although some primitive galliformes might not be much further).
If anyone wants further analogies from me (which I doubt), probably best to do it off list. And my afternoon break is now over, so I bid you Adieu.
------Ken
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