[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

Re: "Cope's Rule" Put to the Test



                WAGNER's STATEMENT ON COPE'S RULE


GIVEN:
        1) Selection pressure drives adaptation
        2) Adaptive radiations do occur

POSTULATED:
        1) Basal members, and by implication ancestral members, of adaptive
radiations
                seem to be small animals (agreement with George).  This is
only taken
                here to apply to tetrapods.  Not that this is possible, not
proven.

NOTED:
        1) We seem to mostly be concerned with adaptive radiations into
large animal
                niches.  This may be due to:
                a) Preservational bias
                b) Observational bias (the horse example is prevalent).
Major adaptive
                        radiations occur after an extinction of large forms,
and the
                        evolution of new large forms facinates us.
                c) Analytical error
                Note that, in any case, we may end up ignoring smaller
members of a
                        radiation.

        2) Adaptive radiation into large animal niches tautologically
demands a size
                increase, if one begins with a small ancestor.

EXPOSITION:
        1) Adaptive radiations into *large animal niches* involve increase
in size.
                a) This may be partially a product of the hypothetical small
size
                        of the ancestral group.
                b) At some point in the adaptive radiation, it is certainly
concievable
                        that net selection pressure for an increase in size
will exist.
                        This is especially applicable when large animal
niches are not
                        occupied.

        2) There is nothing to say that an increase in size must be gradual
or constant.
                Each species experiences differing selection pressures, and
the size of
                a putatively decendant species is not dependant on that of it's
                predecessor.

        3) There is no a priori reason for a size increase.  It is purely due to
                selection pressure.

        4) There is no a priori constraint on size decrease.

        5) There is no a priori reason for size to increase sucessively in
succesive
                species of a lineage.

REFINEMENT:
        1) There is nothing to say that a particular radiation must increase
in size,
                simply that, in the absence of competition, a new radiation will
                probably experience pressure to exploit every niche possible.

                        a) It is certainly possible that these new niches might
                                invlove larger body size
                        ex: It behooves the horse to become larger and
exploit the
                        vast grasslands.

                        b) It is also concievable that there may commonly exist
                                selection pressures in animals to get
larger, especially
                                in those at the top of the size range for
thie fauna.
                                Such selection pressure might include prey/food
                                adapting to counter smaller feeders, or
size-based
                                defensive tactics.

        2) A large part of the demonstration that a generalized increase in
size is
                conditionally based on unproven assumptions.
                a) The evidence of this trend is, and has been, called into
question.
                b) Indeed, a model for why this trend should continue to the
exlusion of
                        size decrease is also lacking, except on a
case-by-case, 
                        historical-narrative basis, which as Bock (1985, in
_The Beginnings of Birds_) points out, is not generally 
                        applicable a priori to new situations.

CONCLUSION:
        Statement of Cope's Law:
                In adaptive radiations of tetrapod clades from small
ancestral stock, clades may evolve to expoit large animal niches, and in
doing so may experience a increase in size due to selection pressure.

        This increase in size is not necessarily regular nor is it
predictable , and it is based soleley on selective pressure.  There is no a
priori reason why selection pressure may not select for smaller animals.
The fact that we apparently observe a trend in size increase may due to any
number of errors, but it is concievably a consequence of radiation in the
presence of available large animal niches.

        As George has amply demonstrated, it is a posteriori possible to
explain selection pressure.  However, as selection pressure is so
case-specific, it is not necessarily possible to apply observations of
selective pressure and their posteriori explanations to a different
situation a priori (also see Bock 1985, in _The Beginnings of Birds_).

        To bring this back to my original point, Cope's Rule is at best a
generalization, and cannot be used to support or refute the validity of
"BCF".  In the context of case-specific selection pressure, there is nothing
a priori less valid about assuming that coelurosaurs may have undergone a
"size-squeeze" before birds evolved.  There is nothing a priori more valid
in insisting that non-avian dinosaurs (as we know them) were too large to
evolve directly into birds.

        QED

        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             |
+--------------------------------------------------------------------------+