Re: Long branch attraction

[David Marjanovic <david.marjanovic@gmx.at>]

----- Original Message -----
From: "Phillip Bigelow" <bigelowp@juno.com>
Sent: Monday, August 14, 2006 6:23 AM

> I would appreciate a simple explanation for the concept
> of "long branch attraction".

A long branch is the result of a high mutation rate (branch lengths are 
measured in numbers of substitutions). When so many mutations have happened, 
there's a good probability that synapomorphies which the taxon in question 
might share with anything else are overwritten by more recent mutations, 
leaving only convergences that cladistics can use as evidence for 
relationships. Thus, long branches tend to be found as sister-groups of each 
other -- the longer two branches are, the more likely are they to share 
homoplasies.

If two taxa with long branches are in reality sister-groups, you get the 
opposite effect -- long-branch repulsion -- for the same reason -- the 
synapomorphies are overwritten.

Turtles and alvarezsaurids are examples for long branches in morphology: 
they are so modified that it's hard to figure out what their sister-group 
is -- too many potentially useful characters have a unique state or are 
inapplicable.

> "If you are traveling into the future in a time machine, and you pass a
> person from the future who is traveling into the past, it's probably a
> good idea to avoid eye contact." - Jack Handey

Why?

(Apart from the concept of "you pass" requiring space, not time.)