Peter Ruest writes:
[big snip...]
>3) The homology of vertebrate limbs: the main problem with all
>similarities between functional features is that they may need to be
>similar in order to function: thus, they cannot be evidence for common
>descent.
I would agree with this if a singular component was being used for comparison.
For instance, there are examples of enzyme active sites where strong arguments
can be made for convergence rather than homology. However, when the item
being considered for homology is itself composed of multiple components or
features, as vertebrate limbs are, then I think similarities of the component
parts can be used to make a legitimate case for common descent. I think that
with more components available to produce a "function" there would tend to
be more ways to produce that functionality, particularly in macroscopic systems
such as limbs. That would tend to undercut the argument for "similar functions
requiring similar forms".
>Genuine evidence for homology may require functionless features (cf. my "How
>has life and its diversity been produced", PSCF 44 (2/1992), 80-94).
[...]
That's definitely one strong class of evidence for homology. And I think it's
certainly a legitimate concern in molecular biology for studies about the
origin
of single enzyme domains or protein families. But for multi-enzyme or higher
level, multi-component systems that are shared across different species, it
can be harder to make the case for convergence due to functional necessity,
particularly if the ultimate functions are not quite the same across species
(i.e. Consider the many end-functions of vertebrate limbs in various species:
terrestrial locomotion (quad- and bipedal movement), aquatic propulsion,
grasping, ripping, mating, cleaning, burrowing, communication, & etc.)
Regards,
Tim Ikeda
tikeda@sprintmail.com
This archive was generated by hypermail 2b29 : Mon May 28 2001 - 21:50:06 EDT