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PLEASE NOTE: On a related topic



Jarno Peschier (just picking on you, dear fellow) wrote:

>Since _Baryonyx_ is suspected by some to be related to _Spinosaurus_
>and _Irritator_ (right?), [...]

        Lest we should all forget, _Baryonyx_ most certainly *is* related to
_Spinosaurus_. I think everyone on this list will agree that the evidence
for a common ancestry for vertebrates, if not for all life on Earth, is
fairly impressive and quite convincing (I'm not going to get mired in this
"scientific truth" stuff). The real question is, "how closely related are
they", and this question can only be asked in relation to other taxa.

        My own observations lead me to the conclusion that when people say
two taxa are "not related", they are speaking within the context of
traditional/Linnean/"Evolutionary" taxonomy, where "unrelated" animals are
not members of a taxon of a certain rank. The taxon rank in question is
dependant on the speaker's predelictions, the speaker's point of view, and
the context. In addition, the arrangement of taxa heirarchichally is subject
to the analysis of the taxonomist and does not necessarily relate to the
biological processes of evolution. The statement is therefore not only
meaningless literally, but also objectively useless even if we ignore this
fact. I would STRONGLY urge people to abandon this usage, as it is
uninformative, and potentially misleading. 

        Traditional taxa are not evolutionary units, and their use as such
can lead to confusion. Example: Since _Archaeopteryx_ is a descendant,
perhaps a direct descendant, of Superorder(?) Dinosauria sensu antiquus, but
is a member of Class Aves, is it "related" or "closely related" to
dinosaurs? If you say it is, what about _Confuciusornis_? Or a member of
Order Enantiornithes? Some would say they "are not related", some would say
they are. Some say the Cheetah is not related to other cats because it is a
different genus. Hmmm? Some say some antelope are not related to others
because they are in a different family. But the species, its genus, and its
family may be a direct descendant of a member of that other family.

        "Closely related" is almost as egregious. How does one assess
"relatedness" in an objective way? Presumably, this was traditionally
supposed to be delineated by membership in a common taxon of a certain rank.
We have already seen that this does not work. What about number of character
transformations (branch length)? This wasn't that popular, last time I
checked. Genetic similarity? I don't know too much about this, but I haven't
heard a proposal for using it to directly quantify "relatedness" in an
absolute sense.
        The only objective, scientifically meaningful way that I know of for
determining "relatedness" is propinquity of descent (recency of common
ancestry). Thus, while two taxa being "related" is a given, two taxa being
"closely related" can only be evaluated relatively; that is, with respect to
a third taxon. For example, "A is more closely related to B than to C,"
implying "A shares a more recent common ancestor with B than with C." Given
a phylogenetic hypothesis, statements made in this manner are
phylogenetically meaningful, scientifically testable, and easily conceptualized.
        "A is closely related to C" can only have meaning when given an
explicit context, such as "within group B", and even then it is vague.
However, at least then you have narrowed the scope to the level where others
have a chance of understanding what you are saying. With the advent of
phylogenetic taxonomy, where taxa have an explicit phylogenetic meaning, we
can now avoid this altogether with statements such as "A and B are both
members of group C". Given a limitied knowledge of the local tree topology,
this can convey just as much information in an explicit manner.

        UPSHOT: "_Baryonyx_, _Spinosaurus_, _Sigillissimasaurus_ and
_Irritator_ (and possibly _Angataurama_) are suspected of forming a clade
exclusive of all other known theropods." Explicit, to the point, unambiguous.


        
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    Jonathan R. Wagner, Dept. of Geosciences, TTU, Lubbock, TX 79409-1053
"Cladism is to evolution what agnosticism is to the existance of god"-Horner