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Olshevsky's Rule, the Critical Response (was Re: "Cope's Rule" Put to the Test)



Dinogeorge@aol.com wrote:
>I use a rather different form of Cope's rule: Large organisms tend to evolve
>from smaller organisms rather than vice versa. The version of Cope's rule in
>parentheses above is obviously false and never needed any kind of 10-year
>study for disproof.
        Unfortunately, the article was no longer available (if anyone knows
how to access archives of those articles, please e-mail me privately).
        In any case, George's restatement of Cope's Rule is still not a
useful generalization.  I would like to see some sort of evidence,
preferrably backed up by a theoretical model, for this theory.  I cannot
accept evidence from phylogenies which accept this rule a priori, what I
would like to see is a statistical analysis using cladistic techniques.
        (It should be born in mind that there is some objection in from the
field of biometrics (Strauss, pers. com.) concerning cladists who end up
coding several size-related characters seperately, thus overwieghting size
and potentially producing clusters of larger taxa and obscuring
relationships vis a vis small taxa.)
        Ironically, if done correctly, this is exactly the proof needed to
prove the original statement of Cope's rule, and the article in question
supposedly shows no clear evidence in support of either conclusion.
        So I also ask for a model of how this would work.  George has, in
the past and in the context of his BCF theory, seemed to imply that there is
some inherent evolutionary mechanism blocking size reduction.  He has used
"evidence" of the sort used to support the original statement of Cope's
rule, thereby implying that because clades trend towards large size, there
must be something blocking large size.  He has also mentioned that dwarfism
(the only clear cases we have of of large begetting small) is an exception,
frequent caused by reproductive isolation and competition for scarce resources.
        I would point out that these latter influences are exactly the sort
commonly described as causing or contributing to speciation.  The trend
towards larger size over time has been called into question, and George
himself says it never needed disproof.  So we then are left wondering what
causes George to reach his conclusion.
        What we need to be looking for is the source of SELECTION PRESSURE.
Several persons (none the least of them Bakker and Paul) have pointed out
the inherent advantages in large size.  Certainly, in the absence of other
pressures, it may be reasonable to assume that large size could be strongly
selected for.
        However, small size is also advantageous (as John Bois has attempted
to demonstrate on this list).  A mechanism by which small size may be
achieved is easily derived.  Indeed, it may involve LESS overall adaptation
than an increase in size.  Due to our terrestrial (and not Ork-ian) origin,
all tetrapods pass through ontogenetic stages where they are SMALLER than
the adult for of their species.  let me not understate the possible changes
necessary to convert such a paedomorphic animal into a fully functional
adult, but, suffice it to say, the size is already there.
        The percieved rarity of smaller decendants of larger animals may be
due to preservational bias, poorly thought out character selection, or any
number of other possibilities.  It hardly matters.
        Often, perhaps even more often than not, ceteris paribus, there may
be greater selection pressure for large size than small.  Yet, as Mickey has
gone to great lengths to point out on this list, evolution is not geometry,
and one cannot use this supposed trend in a phylogenetic argument and expect
to be taken seriously.
        So we are left with the conclusion that selection pressures are
different on different groups at different times, and a blanket statement
such as the one George makes is above is useless (not to mention difficult
to prove), especially for the purpose it serves in his BCF theory.  George
tries to use this as evidence that the "too large" non-avian dinosaurs could
not have given rise to the smaller birds.
        Perhaps George has never heard of the hummingbird, which is quite a
bite smaller than the earliest known tetrapod, smaller the the earliest
known aminotes, smaller than any dinosaurian outgroup, indeed quite a bit
smaller than _Archaeopteryx_.

        Wagner

        
+--------------------------------------------------------------------------+
| Jonathan R. Wagner                    "You can clade if you want to,     |
| Department of Geosciences              You can leave your friends behind |
| Texas Tech University                  Because your friends don't clade  |
| Lubbock, TX 79409                               and if they don't clade, |
|       *** wagner@ttu.edu ***           Then they're no friends of mine." |
|           Web Page:  http://faraday.clas.virginia.edu/~jrw6f             |
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