Farewell to Species - reticulation

A Mitchell am16 at GPU.SRV.UALBERTA.CA
Tue Feb 1 13:35:15 CST 2000

Thomas DiBenedetto wrote:

>>Thomas Lammers wrote:
>>Submitted for mulling over: cladistics is flawed science because it comes
>>to a conclusion first ("all evolution is dichotomously divergent, with
>>ancestors becoming extinct at the appearance of a pair of daughter-taxa")
>>and then seeks data to support that conclusion.  When the data do not
>>support that conclusion but suggest that other patterns are operative and
>>equally valid (e.g., when we get polytomies or reticulations), they are
>>judged to be faulty and dismissed.
>I disagree with just about all of this.
>First of all, "all evolution is dichotomously divergent" is neither an
>assumption of cladistics, nor is it a conclusion. Cladistics can and does
>proceed without assuming this (hard polytomies are an accepted and
>recognized concept amongst cladists). And cladistics does not present this
>as a conclusion; once again, polytomies are found in most published
>cladograms, and they are interpreted as saying "there is no evidence for a
>dichotomous representation in this case". Although some cladists may be
>motivated to continue their research because of a personal feeling that
>there just must be some evidence out there to resolve the polytomy, this
>does not lead to a biased result, since no such resolution will be presented
>unless the evidence is actually found.
...and lots more about polytomies <snipped>

I agree with just about everything you said Tom, but what have polytomies
got to do with reticulation?  For a cladogram to accurately track a
reticulation event it will have to show two lineages fusing into one, e.g.:
in the tree below an offshoot of the lineage leading to B and C hybridizes
with an offshoot of the lineage leading to D.  Ever seen Hennig86 or PAUP
produce a tree like this?

    A   B     C   Z       D
    \   \    /    |      /
     \   \  /   __|_    /
      \   \/   /    \  /
       \   \  /      \/
        \   \/       /
         \   \      /
          \___\    /
               \  /


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Andrew Mitchell                        _____     \ /     _____
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