Talk about synchronicity. A few minutes after I replied to a post on
choosing a model with LRTs, the new Systematic Biology arrived in my
mailbox, including a paper that explains everything:
Posada, D., and K. A. Crandall. 2001. Selecting the best fit model of
nucleotide substitution. Syst. Biol. 50:580-601.
*Note the obvious spam-defeating modification
to my address if you reply by email.