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Re: Dinosauria---Rejected Name?
firstname.lastname@example.org (whatever happened to names? :) wrote,
As the author of the post David Marjanovic referred to, I thought I should
throw in a comment or two, but we *ought* to get back to dinosaurs...
> So, 'a clade is not a group, it is a monophyletic entity...'. I suspect
> means that the composition of the clade may be changed without changing
No, I think you missed the point. The absolute composition of a clade
does not change with analysis, just *our hypothesis* of the clade's content.
Holtz named Arctometatarsalia, and hypothesized that it included o-mimids,
tyrannosaurs and trodontids. He now suggests the latter two groups are not
members of the clade he named. The TRUE content of the historical entity
Arctometatarsalia has not changed, but our hypothesis of its content has
changed, based on exclusive groupings recovered from phylogenetic analysis.
Clade content can change, however, when included species speciate... hard to
observe in a conventional research program.
> If so, the entity is a construct built more on expectation than
> observation; if the entity weren't expected to continue to exist, it could
> be disbanded.
The entity is hypothesizd to exist, based on the evolutionary theory
that all organisms share common ancestry. From this, it naturally follows
that any two organisms share a most recent common ancestor. The ancestor
exists, the clade exists... if you wish to reject this, you will have to
assert that all life does not share a common ancestry.
> Also, 'clades are indicated by an anlysis, not fully discovered.' and
> 'clades are named with the intention of discovering the known descendants
> the specified common ancestor.'
> The supposed 'descendants' of an actually hypothetical ancestor are
The ancestor is absolutely, 100% NOT hypothetical, in the absolute
sense. Again, if you wish to doubt that giraffes and beetles share a common
ancestor (many, actually), you will have to account for a termendous number
of convergences (e.g., they both have mitochondria, both share DNA encoded
using an identical or nearly identical amino acid code, both have
chromosomes, etc.). Now, that ancestor's relationships to other species IS a
matter of hypothesis and test.
> because of an analysis, an inference which may not be correct ('not fully
Their inclusion is an hypothesis. We SUGGEST that they belong to the
> Then '...clades are named with the intention of discovering
> the known descendants of the specified common ancestor.' So, as I
> understand it, the intention to do further analysis qualifies the analyst
> name his/her hypothetical entities for general usage.
Again, the entity (clade) is NOT hypothetical. From the theory of common
ancestry, we can assert that any two organisms share a common ancestor. We
name that clade, we do NOT hypothesize its EXISTANCE, just its content.
Let's just review: "the most recent common ancestor of Triceratops and
Ornitholestes and all of its descendants" (a phylogenetic definition for a
taxon name, one currently not being employed) exists, independent of our
ability to recognize its diagnosis, content (other than the taxa used to
delimit it), internal structure, or any of its other inherent
particularities. You may think this includes all of the taxa traditionally
termed "dinosaurs" and "birds," I may think the only known members are the
specifying taxa, Hypsilophodon foxii and three of the species of
Psittacosaurus. Those are hypotheses of its content; neither says anything
about whether it exists or not, and nor does the fact that it was named in
the context of either of these possible contents (or any other). If you can
come up with circumstances under which these two taxa do NOT share a common
ancestor, then maybe we can talk.
> Finally, though the analysis is not necessarily complete and correct ('not
> fully discovered') the author says, 'the natural ordering of life (through
> phylogenetic relationships) is discovered...' That's a very definite
> statement which appears to contradict some prior statements.
You took this out of context, and clipped the end of the last quote:
"and clades are named with the intention of discovering the known
descendants of the specified common ancestor." This statement was part of a
contrast with traditional taxonomy, in which heirarchichal order is IMPOSED
on the diversity of life, with groups drawn up and circumscribed prior to
naming and analysis (if any). This was meant to emphasize the role of
hypothesis, test and discovery (science) in determining the structure of a
strictly phylogenetic nomenclature. I was certainly not implying that
species placed in a phylogenetic taxonomy will naturally ALWAYS fall in
their correct historical groupings. That would be patently silly.
> Just one other quote:
> '...a phylogenetic name ALWAYS refers to the same clade, and therefore the
> same included species. The crux of phylogenetic nomenclature is that the
> included species are HYPOTHESIZED...'
Do you have an (articulate :) problem with this statement? If there is
one true history of life (there is), a name defined phylogenetically will
always refer to the same group of species. We may never know all of them, or
we may never put them under the correct name, but they are there,
independant of our analysis. That's the point of phylogenetic nomenclature:
using the real heirarchical natural of life as a basis for taxonomy. That
way, taxonomic uncertainty reflects real phylogenetic uncertainty, rather
than some combination of this and scholarly fiat.
> Responding to the author's comment, please consider this a brief, partial
> RETAIL observation.
I wonder if Mickey and Mary can time you out for excessive cuteness. ;)
Hope this helps,
Jonathan R. Wagner
9617 Great Hills Trail #1414
Austin, TX 78759