tips vs nodes

Guy Hoelzer hoelzer at unr.edu
Thu Jun 8 11:50:34 EST 2000


In article <8hmopn$l61$1 at mercury.hgmp.mrc.ac.uk>, Dmitri Sherbakov
<sherb at srv01.lin.irk.ru> wrote:

> I came across a dataset of 18S rRNA sequences of algae, where phylogenetic 
> analysis suggests the following:
> 
> A->B->C->D            (1)
> 
> instead of "normal"
> 
> A   B   C   D         (2)
> !   !   !   !
> +-+-+   !   !
>   !     !   !
>   +--+--+   !
>      !      !
>      +------+
> 
> due to zero or almost zero lengths of some branches.
> Since the case (1) may be describe realistic evolutionary events, the
> questions are:
> 
> Is it possible to built a test distinguishing between (1) and (2)?

The only difference between cases (1) and (2) is branch lengths, not
topology; so the test would be one comparing branch lengths.  Relative
rate tests should do the trick.

> Did anybody meet and not discard stuation like that?

The kind of topology you describe is typical of many viruses in nature
(see the papers by Stuart Nichol and Walter Fitch).  It is usually
interpretted as evidence of selection dominated evolution; evolution stops
when the organism becomes optimally adapted to a particular environment
and bursts ahead when some individuals successfully colonize a new
environment.

-- 
Guy Hoelzer
Department of Biology
University of Nevada Reno
Reno, NV  89557








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