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Back again. I'm taking both "Comparative Animal Physiology" and
"Ornithology" this semester, so I plan to study the dinosaur metabolism
debate on my own time. A couple questions first: 1) besides Allosaurus
and the new DNM theropod, how many theropods (yes yes, I mean besides
birds, sigh...) have furcula? 2) Has the paper sinking Ultrasaurus into
Supersaurus been published yet?
I've mulled things over this past month or so, and I've
come up with a few arguments against the "real" nature of clades, and the
value of paraphyltic groups. I'll be discussing archosaurs in at least
ONE example, so this shouldn't be too innaproriate. If the debate gets too
off the dinosaurian track (which I am sure it will), we can always go to
private correspondance. Be warned, this is pretty long.
I. THE DUAL NATURE OF CLADISTICS
As I see it cladistics serves two main functions, one very well, the
other less so. I feel these functions were confused in the debate on
cladistics in November, so I want to spell them out now:
A. PHYLOGENETIC INFERRENCE
Determining in what order genera and species branched off from each
other is what cladistics does well. Make no mistake, I do believe
synapomorphies are the best way to infer phylogenetic relationships.
Drawing the little dotted lines after the phylogenetic tree has been
reconstructed. It is this taxonomic function taht I feel cladistics needs
to augment with paraphyletic groups. More on this later.
II. THE ARBITRARY NATURE OF CLADISTICS
THERE ARE NO "REAL" OR "NATURAL" GROUPS
Species do not naturally aggregate themselves into "groups". The
fact that a species is decended from another does not mean that they must
form a "group". Saying clades are "natural" is a bit like saying because
the border of Australia is defined by a natural coastline, it must mean
Australia is a "natural" country (I know I've probably grossly
oversimplified Australia's territorial boundries, but you get the idea).
As people intrerested in paleontology, biology, and zoology, we may PREFER
to group organisms together based on phylogenetic considerations, but this
does not make our groupings, regardless of if they are monophyletic,
paraphyletic, or polyphyletic, more "real" than if we classified them
based on average size or numbers of individuals. How could any grouping
of species, regardless of the criteria, ever be "real"? CLADISTS set the
rules that groups must begin with a node and end with extinction (or the
present day- more on that later), not Nature. How is common descent any
more real than the possession of scales or the absense of hair, or the
endothermic or ectothermic metabolism? Cladistic rules may limit the
potential number of groups more that paraphyly, but it doesn't make these
groups any more real.
ANCESTRAL SPECIES ARE PARAPHYLETIC
Yes, I know identifying bona-fide ancestral species in the fossil
record with 100% certainty is impossible. That is irrelevant: they
existed, and unless the offspring of every single individual born within
that species survived to reproduce, these species MUST be paraphyletic.
PARAPHYLY IS AN ILLUSION OF WHERE THE CLADIST STANDS IN TIME
Every single paraphyleic group that exists, as diagnosed in the
Linnaean sense, WAS ONCE MONOPHYLETIC. Before the evolution of tetrapods,
ray and lobe finned fish could have been united in a perfectly good
monophyletic clade. Before the evolution of therapsids, the same was true
for "reptiles" (in the Linnaean, not Gauthier sense). If clades are
"real" and paraphyletic groups are not, what does that mean for fish and
reptiles? They used to be real, but now they're not? Is the only reason
monophyletic groups are "real" and paraphyletic groups are not is because
WE happen to be living at this particular point in time? How "real"
a system of classification is that?
III. THE LEGITAMACY AND VALUE AND OF PARAPHYLETIC GROUPS
EVERY APOMORPHY DEVELOPS AT THE EXPENSE OF A PLESIOMORPHY
Paraphyletic clades are therefore NOT just definable by the absense
of an apomorphy, but by the PRESENCE of a trait no longer possessed by the
descendant group. Yes, I know birds and some mammals have scales, but
everywhere they HAVE feathers or hair they are LACKING scales. Reptilia
could be redefined in part as a group POSSESSING scales over these
particular areas). Unless it matrilized out of thin air EVERY apomorphy
develops at the expense of an ancestral trait through modification of an
existing feature. Once the new form of that feature develops, the old
form of that feature ceases to exist. The fact that a descent group
possesses the apomorphy is no more real then the fact that the ancestor
has the ancestral trait.
"BASAL" GROUPS- EVEN CLADISTS USE PARAPHYLY WHEN IT IS CONVENIENT
One example is Paul Sereno's paper on "Basal Archosaurs". He
mentions ankle bone development in the earliest dinosaurs, pterosaurs,
and crocodilians, but doesn't pursue its further development in these
groups. However, he does go into some detail about every other
monophyletic archosaurian clade that didn't make it out of the Triassic.
All Archosaurs except dinosaurs, pterosaurs and crocodilians: what group
is paper really about? (I'll give you a hint- it starts with a "t").
ALL MEMBERS OF A MONOPHYLETIC CLADE DO NOT NECCESSARILY HAVE MORE SHARED
DERIVED CHARACTERS IN COMMON WITH EACH OTHER THEN THEY DO WITH SEPERATE
Monophyletic clades are defined by common descent, not greater
similarity. For an example, look at this cladogram:
species A species B species D species C
* * * *
* * * *
* * * X
* * 3
* * *
A series of apomorhies defines clade 1 that are not present in the
outgroup (not shown). Additionally, clades 2 and 3 are monophyletic.
Species D may very well share more than it does apomorphies (for clade 3)
with species C. This is arguably the case with the reptiles (Linnaean-A,
B) "basal" synapsids (C) and mammals (D), and also with fish (ray fins A
and B), Lobe fin C, and tetrapod D. In all cases, A, B, and D share
important aspects of behavior, metablism, anatomy, and/or physiology than
D does with C.
If the only aspect of evolution we were interested in was "who is
descended from whom", this would be perfectly good, but in the words of
Ernst Mayer, cladisitics "denies other aspects of evolutionary change such
as the rate of evolution, adaptive radiation, the occupation of new
adaptive zones, mosaic evolution, and other macroevolutionary phenomena".
These things may be (somewhat) separate from phylogeny, but
why do it have to be seperate from taxonomy? As evolutionists, we are
interested in these subjects. Who said, and WHY, that phylogenetic
relationships were the be all and end all of taxonomy?
And lets not hear any more crap about paraphyletic groups only having
value to creationists or people wanting to cast moral aspirations on
certain animal groups.
"Or one could make the case that the world has already ended, that
art is dead. Instead of art we have pop culture, day time TV: gay senior
citizens, women who have been raped by thier dentists confiding in Oprah,
an exploration in depth of why women cut off thier husband's penis."
"Well! Time to fill up a glass."
- Re: Clade II
- From: Jeffrey Martz <martz@holly.ColoState.EDU>
- Re: Clade II
- From: Nick Longrich <firstname.lastname@example.org>
- Re: Clade II
- From: Gautam Majumdar <email@example.com>