Re: Digit Loss

[Daniel Bensen <dbensen@gotnet.net>]

>> What is interesting is that some theropods have lost two digits, and these 
>> digits never seem to re-evolve.  This may mean that the evolutionary 
>> pressure still existed, preventing the re-evolution of the primitive state.  
>> <<
    I disagree here.  Remember that evolution is driven by random mutation.  To 
loose a digit or limb is a relatively simple mutation: fail to turn on a switch 
during development, and the embryonic digit is starved for nutrients, or the 
proto-digit cells fail to propagate, or some other pathway is blocked.  To 
create an entire digit out of whole cloth, however, is quite another matter.  
Once a gene is deleted, it's gone.  Well, with all the redundancies of the 
genome, primitive genetic material may exist somewhere, but the series of 
mutations necessary to bring an old sequence back into expression is probably 
no easier than
creating the genes from scratch.  A random mutation, falling anywhere in the 
genome, is much more likely to destroy a digit than to create one.
    We see numerous examples of digit-loss in tetrapod evolution, but no digit 
creation.  The closest instance I can think of to digit creation (except for 
aberrant things like two pinkies---which don't seem to propagate through 
breeding(has anyone looked into that?)) is the panda's thumb, actually an 
altered set of wrist bones, that went through all sorts of intermediate 
mutations before arrive at its present, functional state.  Any seeming tendency 
for digit-reduction is due to the fact that more-toes mutations are much harder 
to achieve than less-toes mutations, and so the less-toes mutations tend to 
build up over time.


    For most animals, digit-loss would represent a neutral mutation, or perhaps 
a mutation of slight benefit (as nutrients that might have gone to nourish a 
lost digit can now be diverted elsewhere), so, in most cases, a mutation that 
causes the loss of a digit should be preserved, not because of any particular 
advantage of a small number of digits, but because of the general sloppiness of 
natural selection.  Ostriches (or Hesperorni) are(were) flightless, not because 
small wings are good, but because large wings are bad (they are divert energy 
from other, more necessary appendages).
    To get to the point, digit number in dinosaur clades represents a series of 
trivial mutations, responding to very weak selective pressures, just the sort 
of thing that is so useful in classification.  As Darwin said in _The Origin of 
Species_ (I paraphrase because I can't seem to find my copy), these trivial 
details (openings in the skull, bumps on molars, or number of toes), respond to 
no particular selective pressures and so vary more or less randomly through 
time, making them useful in determining phylogenies.  We cannot safely classify 
dinosaurs based upon the shape of their claws (as the selective pressures that
govern these implements are strong), but we can use number of digits.

Dan