Hi Tim,
Thank you for your comments!
Tim Ikeda wrote:
> 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"
You are right. But there is one more problem: similar cladistic trees
may be a consequence of the features concerned being interdependent. The
function of limbs is certainly a consequence of the functions of many
component parts. Now, if two components are functionally interdependent,
their evolution is interdependent, and we would expect the two cladistic
trees formed for the two features in a group of taxa to be similar, no
matter how this came about. Just as a similarity between the same
feature in two taxa may be a consequence of either common descent or
common requirements, so a similarity between the cladistic trees
constructed for two component features in the same group of taxa may be
a consequence of either a descent acording to the cladistic tree found
or of the two components being interdependent. This case has also been
described in PSCF 44 (2/1992), 80-94. Of course, if the compound
function under consideration has been formed in different ways in
different taxa, there will be at least some different interdependencies
between the component functions or features, and there might be a
stronger argument for evolution.
> >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.)
My argument given above includes the species-specific modifications of a
given structure or function. If a given requirement is similar in two
species, we expect the corresponding structures in the two species to be
similar. A natural corollary of this is that the differences between
these requirements in the two species will lead us to expect
corresponding differences between the two structures, no matter how this
state of affairs came into being. We would expect limb buds to be very
similar between frogs, ducks, dogs, and dolphins, but the further they
advance in their embryonic development, the more they will differ from
each other in order to function properly. The same would apply to any
other multi-component system.
> Regards,
> Tim Ikeda
Peter
-- -------------------------------------------------------------- Dr Peter Ruest Biochemistry Wagerten Creation and evolution CH-3148 Lanzenhaeusern Tel.: ++41 31 731 1055 Switzerland E-mail: <pruest@dplanet.ch - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - In biology - there's no free lunch - and no information without an adequate source. In Christ - there is free and limitless grace - for those of a contrite heart. --------------------------------------------------------------
This archive was generated by hypermail 2b29 : Wed May 30 2001 - 11:06:38 EDT