Farewell to Species - reticulation

Hubert Turner turner at RULSFB.LEIDENUNIV.NL
Tue Feb 1 23:24:32 CST 2000


On Tue, 01 Feb 2000 13:35:15 -0700, A. Mitchell wrote:

>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
>    \   \    /    |      /
>     \   \  /   __|_    /
>      \   \/   /    \  /
>       \   \  /      \/
>        \   \/       /
>         \   \      /
>          \___\    /
>               \  /
>                \/
>                /

Because permanent splits are the only things recognized by a
cladistic analysis,  in the ideal case (i.e. when no homoplasies
occur to muddle the outcome of the analysis) the cladogram would
be:

A   B  C  Z     D
 \   \/   |    /
  \   \   |   /
   \   \  |  /
    \   \ | /
     \   \|/
      \   /
       \ /
        /
       /

because apomorphies cannot be shared by common descent by only Z
and D, or by only (B, C) and Z, or by only (B, C) and D. The whole
lineage between the branchin off of taxon A, and the 'hybridization
event' leading to taxon Z, is not permanently split, and hence must
be considered a single unit, which splits polytomously [sic!] into
(B, C), D, and Z.
A propos polytomies, of course situations like these, where
there is no information to reconstruct the temporal sequence of the
splits leading to (B, C), D, and Z, occur regularly. That is not to
say that (B, C) did not split off befor D and Z broke apart, only
that there are no synapomorphies to reconstruct the actual sequence
of events. Think of (B, C) as a e.g. a sister pair of divergent spiders in
Great Britain and Ireland, resp., and D a divergent spider in Taiwan, with
Z the widespread third branch (possibly indistinguishable from the common
ancestor).

Hubert Turner


*******************************************************
Dr. Hubert Turner
EEW, Sect. Theoretical Biology & Phylogenetics
PO Box 9516, 2300 RA  Leiden, The Netherlands
Visiting address: Van der Klaauw Laboratory, Kaiserstraat 63, Leiden
Phone: +31-71-5274904    Fax: +31-71-5274900
E-mail: turner at rulsfb.leidenuniv.nl
WWW: http://wwwbio.leidenuniv.nl/~turner/index.html

FROM 18 JANUARY TILL MID-APRIL I WILL BE VISITING THE NEW YORK
BOTANICAL GARDEN AS VISITING SCHOLAR, DOING MOLECULAR
PHYLOGENETIC RESEARCH ON ANACARDIACEAE. THIS E-MAIL ADDRESS
WILL REMAIN FUNCTIONAL DURING THAT PERIOD.
*******************************************************




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