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Re: Coracoid/scapula (was Re: flying Archie)
Hmmmmmm. Actually, if you take underwater 'flight' as a functional equivalent
of flight proper (and I do), that sounds fairly hard-and-fast. Thanks.
Don't know anything about the present topic of this thread, but as to
"reversals/re-reversals" generally-- it has long been my feeling that
convergence is usually the parsimonious scenario, as it is more likely for a
fortuitous event to happen once to several (eg, 3) individual species (out of a
larger "pre-cursor space"), than it is for 3 fortuitous events to happen to one
species. Although Tim is right, there is maybe no way to tell...
Oversimplified to the max-- If the probability (p) of evolving a trait (a) = x,
and the probability of reversal (b) is y, then for one species the p(a to b to
a) = yx^2. The p(a) for 3 separate species on the other hand, does _not_ equal
x^3, unless the entire extant pre-cursor species population for whom trait a is
possible is limited to those 3 species. Even then, x^3 will be smaller than
yx^2 only if y > x (which I grant you, it probably is).
In other words, the p(a) occuring at least once in an entire population of
candidate pre-cursor species (m) is 1-[(1-x)^m]. Even if y approaches 1, the
foregoing number is likely to be larger than p(a to b to a)= yx^2, where m is a
large number and x is much less than 1, as it surely is.
I, uh, thimk.... }: D
----- Original Message ----
From: Tim Williams <firstname.lastname@example.org>
Sent: Tuesday, September 26, 2006 10:42:55 PM
Subject: Re: Coracoid/scapula (was Re: flying Archie)
Don Ohmes wrote:
>If they go flightless, do they always lose the joint?
There's no hard-and-fast rule on this. Big flightless birds (like ratites)
tend to show coossification of the scapula and coracoid. But these guys
have been flightless for a long time, and the wings are pretty much
useless. One group, the dinornithids (moas), actually lost the forelimbs
completely. (Ernest Hemingway wrote a book about this: "Farewell to Arms".)
Birds that are recently flightless, and/or still use their forelimbs for
locomotion, tend to retain a mobile joint. Penguins do, and so do
flightless grebes, both of which use the wings in subaquatic locomotion.
David Marjanovic wrote:
>Ah, then we aren't talking of exactly the same character. I am talking of
>the joint, which I interpret as having 3 possible states: flat
>(plesiomorphic), coracoid in scapula, or scapula in
>coracoid. I may be wrong in which of these is the plesiomorphy, but the
>reversals I mentioned all require independent losses of the joint.
But if the joint is closed, we don't know if any one of these characters is
reversed. The fusion blots the original character state out.