Phylogenetic trees.

A. Contreras-Ramos atilano at IBIOLOGIA.UNAM.MX
Wed Feb 26 15:13:20 CST 1997


>Alan Harvey wrote:
>>What I find curious is the notion that character polarity (and ordering,
>>for that matter) should be done _prior_ to the phylogenetic analysis. This
>>is tantamount to saying that we don't know the evolutionary history of the
>>taxa (else why would we be doing the phylogenetic analysis?), but we _do_
>>know the evolutionary history of the characters that constitute the taxa
>>(else how could we provide a priori polarities and ordering?).

>Tom DiBenedetto wrote:
>Interesting point,,,but, perhaps it is better seen like this,,,by
>using outgroups we assert, or assume, that we know both the character
>and the taxic history at some particular (relatively high) level of
>generality,,,but we do not necessarily know either within the
>ingroup.
>Character polarities imposed through the outgroup criterea rest on
>the assertions that certain taxa are close relatives, and certain
>states are ancestral to the ingroup,,,,even though ingroup polarities
>are thus determined, one really only is making an assumption about
>the condition outside of the ingroup.

Another comment:  It is conceivable that ingroup taxa that are basal or
near basal (i.e., at or close to the outgroup node) may actually be
closer to the outgroup.  This could be suggested by several character
changes excluding these taxa from belonging anywhere further up in the
tree, as well as by characters (which have no intrinsic reason for
potentially not being apomorphous) present both in the outgroup and in
these basal ingroup taxa.  Hence the importance of further and wider
analysis (i.e., more and more outgroups in order to determine direction
of character change at the former OG node).

Atilano Contreras
Instituto de Biologia, UNAM, Mexico.




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