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Re: Cladistics question-hybrids



Betty Cunningham wrote:
>after hearing on the news of the recent camel/llama cross, I'm suddenly
>wondering how cladistics would show such an animal if it could have
>offspring.
        It is important to bear in mind what a cladogram is and is not.
        A cladogram is a branching diagram representing the most
parsimonious distribution of derived characters within a set of taxa. We may
use this distribution as an hypothesis of the evolutionary propinquity of
those organisms. That is, we can use it to formulate hypotheses based on the
question "which taxa are more closely related to each other than to other
taxa." While this is an added layer of interpretation, it is consistant with
the principle of parsimony.
        A cladogram is most emphatically *not* a description of the pattern
that evolution took in producing the taxa under study. Within the confines
of "A is more closely related to B than to C" there are a number of possible
actual evolutionary sequences which may have ocurred (George Olshevsky once
counted six for this list). A cladogram does not directly address any of
these options, although ancestral character-state information derived from a
cladistic analysis (an additional level of inference) may be combined with
other data (such as stratigraphic, paleoecological, etc.) to formulate an
hypothesis. This is an even more derived level of inference, and our the
limited terrestrial vertebrate fossil record may impose severe restrictions
on our ability to evaluate such hypotheses. Indeed, it may not be possible
to test some or all of them.
        
        It is therefore important for everyone to bear in mind that the
branching pattern of a cladogram is intended to show the *relative*
relationships among taxa, it is not a true "evolutionary tree" of *how*
those relationships came to be (although in many cases it may indeed reflect
the course of evolution). This is not to say that, say, birds could have
evolved from crocodyles, even though they are closer to non-avian dinosaurs
on a cladogram. The tree structure (crocs, (dinos, birds)) simply does not
rule out the possibility that birds evolved from dinosaurs, or that the
common ancestor of birds and dinosaurs evolved from crocodiles.
        Therefore, in your camellamma example, the tree structure would
(probably) look like this: (camel, cammellamma, llama), a trichotomous
relationship. Why? Because the cammellamma should be equally closely related
to both parent species. In reality, who knows? It is quite possible that
genetic and evolutionary considerations would lead those characteristics of
the hybrid which were coded for analysis to more closely resemble one or the
other parent. Hybrids are a complex problem. I am extremely glad I don't
work on plants...
        :)
        Wagner

        "The truth points to itself" - Kosh
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    Jonathan R. Wagner, Dept. of Geosciences, TTU, Lubbock, TX 79409-1053
        "Why does the universe hate me?" - G'Kar