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the tonight show (my final post on this)
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
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
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