[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]
Re: Syntarsus [longish]
>In a message dated 96-09-10 10:45:21 EDT, email@example.com (Jonathan R.
>> Pardon my tone, but who are are any of us to say how nature may
>> work? We may say that this structure is so complex (can't say that
>> I agree on that, but anyway...) that it is unlikely to have evolved
>> twice, but this must be wieghed with other characters, to determine
>> the most parsimonious evolutionary hypothesis. It is statements
>> like the one made above which guided, and still guide, may very
>> intelligent people (from Romer to Olshevsky) to premature
>> conclusions about the phylogeny of a group because they (the person,
>> not the group) were unwilling to accept that a certain structure
>> *might* be homoplastic (homoplaisic?)(ie. a reversal, or a result of
>> parallel or convergent evolution).
>The three major problems of cladistic analysis are these: (1) the assumption
>that the most parsimonious distribution of characters gives the correct
>phylogeny is not testable;
That is why it is an assumption. The assumption that the phylogeny of Romer
or Colbert or Olshevsky is correct is also untestable under gradistic
The distribution of characters within a taxon, however, is a testable thing.
> (2) the way morphological characters are defined is inherently
No less so then the way they were traditionally used.
However, it is now possible to a) challenge to coded presence or absence of
a character in another's database and b) to show the effect of the addition of
>and (3) no reliable method exists to weigh morphological characters
>against one another (for example, how many skull characters is a
>postcranial character worth? one? one-half? ten?).
There is a vast body of literature on this. The main conclusion: a priori
weighting of character add an unnecessary second level of subjectivity to
character choice and coding.
>There is simply no way around these problems, so most cladists
>ignore them--and happily continue to consume mass quantities of
>grant money producing endless streams of cladograms with their
This is crap, since a good portion of the literature on cladistics, in
journals like Systematic Biology or Cladistics, concerns itself with
attempts to resolve these problems.
>If at most one cladogram for any group is the correct one, virtually all the
>cladograms so generated >must< be wrong.
Yep. How has this changed from the old days?
> How does one pick the right needle from this huge haystack? What
>statistical manipulation of 66 cladograms, at least 65 of which are
>wrong, will miraculously produce the right one?
The best result from a cladistic analysis is the most parsimonious
distribution of derived character states for a particular dataset, be it one
tree, two, or 66. To expect the angels to come forth and point out the
right one is expecting to much of a method of character state searching.
Why should someone speak beyond the data, unless they clearly state "Here I
speak beyond the data, because I like this tree and not these other ones"?
>> By me old Bio 202 training, not lost on me even as I plow through
>> the pleasures of Structural Geology, there's a little joy called
>> preaption (now, I hear, replaced with the concept of exapation,
>> which now has me royally confused). Basically, the idea is that an
>> organism can have the potential to develop a character without
>> actually expressing it, which said potential can be passed to
>> daughter species, which then may or may not express that character(
>> BOY does it sound hokey when it's written out like that).
>This is a tautology. How will you know when the potential to express a
>character is present, except if the character is expressed?? You DARE to
>criticize paleontologists at the same time that you try to pass THIS off as
Perhaps you are confusion "preaption" (a word with which I am unfamiliar),
whose definition may be as hokey as you state, with "exaptation", a much better
defined term. A classic example of an exaptation is the presence of bone
(hydroxlapatite in a collegen matrix) in the skeletons of vertebrates.
Primitively, bone was used for mineral storage and for muscle attachment (as
it still is in all bony vertebrates). When the first vertebrates became land
dwellers, bone was exapted into a gravity support function, which turned out
to work very well (better than cartilage by itself would).
>Since cladistic analysis of dinosaurs is a hopeless, inbred mess, with
>everyone using everyone else's character matrices and so forth,
Funny. We check each others' data. Weird, that.
Makes so much less sense than standing around bubble diagrams going, "hmm,
no, I don't like that one, I'd rather see bubbles like THIS!". :-)
(Of course, we do NOT always accept each other's codings or polarities of
particular character states, as well demonstrated on November 1, 1995...)
And, don't worry, George, I use your characters, too.
>> ...Do a little research, add some other characters (I think there
>> may be some in Holtz 1994), then run it. If you use at least 40
>> characters and get Dilophosaurus and S. kaentakae as a clade, I
>> beleive that both myself and Tim Rowe will probably have a tasty
>> bite of hat. But you will have achieved a laudable scientific goal:
>> testing a theory.
>> Wagner >>
>So your particular convincement-cutoff is 40 characters. Why? Why not 39 or
You've got me on that one...
Thomas R. Holtz, Jr.
Vertebrate Paleontologist Webpage: http://www.geol.umd.edu
Dept. of Geology Email:firstname.lastname@example.org
University of Maryland Phone:301-405-4084
College Park, MD 20742 Fax: 301-314-9661
"There are some who call me... Tim."