Outgroups and polarized character states

Andrew Mitchell moths at Glue.umd.edu
Tue Nov 4 14:15:46 EST 1997

In article <63lehs$654 at net.bio.net>,  <newsmgr at merrimack.edu> wrote:
>Can anyone give me insight as to how an outgroup can polarize my data set
>which is obtained from a monophyletic study group? I am aware that the
>choice of an outgroup is crucial in that it must be too distantly or too
>closely related to the ingroup, but how exactly will the out group polarize
>the character states?
>Thanks for your time

I guess you meant the out group must NOT be too distantly related to the
ingroup (if it is too far away you tend to get random rooting due to "long
branch attraction") but I see no reason why the outgroup shouldn't be very
closely related to the ingroup.  Quite the oppposite in fact, provided you
are sure the ingroup is monophyletic with respect to the outgroup.

How does the outgroup polarize the character states?  Quite simply, the
node joining the outgroup to the ingroup is the most basal node in your
phylogeny.  If you insist on looking at it on a character-by-character
basis, then that state found in the outgroup (assuming there is only one
outgroup taxon) is assumed to the primitive, thus polarizing your states).

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