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Re: Digit loss

<...[I]n certain cases, such as when appendages or organs no longer serve a
function (or lose most of their original function) through selection, they
are no longer selected for traits that improve or regain that function.
[Me:  but they could improve or regain that function by mutation so long as
the capability still existed.]

IMHO, the reduction of tyrannosaurid forelimbs was not a random change. But,
I would aver that the loss of a manual digit (producing a didactyl manus)
did persist by random chance, not by selection.

I'm not necessarily saying that tyrannosaurid forelimbs were utterly
useless.  What I am saying is that whatever function tyrannosaurid hands
were used for, it didn't matter if they had three fingers or two fingers or
one finger.  A loss of a finger (from three to two) was not detrimental to
the animal's fitness.  This trait was retained not because it was good
(beneficial) but because it was neutral (had no effect on the tyrannosaur,
one way or the other).>

Agreeing, mostly.  The consistent direction of reduced forelimbs necessary
to make them look so vestigial does argue that there must have been
selective pressure.  I do wonder, though, how the loss of capability would
be a benefit.  Unless someone wants to argue 'use it or lose it', the fact
that the forelimbs were so far reduced means that a tyrannosaur would have
had less flexibility if the environment changed.  Therefore, not being able
to use the forelimbs so extensively was achieved at a price less than the
gain.  Hard to figure what such a significant gain might be.
As an aside, 'use it or lose it' does imply a directionality in evolution.
Unless there is a significant functional gain, the only advantage to losing
it would be aesthetic.
On the number of fingers, if there were no advantage to a specific number,
and if mutation were neutral and fairly extensive, wouldn't you expect to
see a different number within the same species coming from the same time
period?  And among species?  You could also expect to see the digits
disappear and reappear, much as 6 toed cats have 5 toed offspring.
Until variation appears extensively in the record, isn't it most appropriate
to think that at some point in the past there was an advantage to a certain
number of digits and that the capability of re-expressing lost digits had