Suggested citations:
Yang, Z. 1997. PAML: a program package for phylogenetic analysis by
maximum likelihood Computer Applications in BioSciences 13:555-556.
Yang, Z. 2007. PAML 4: a program package for phylogenetic analysis by
maximum likelihood. Molecular Biology and Evolution 24: 1586-1591
(http://abacus.gene.ucl.ac.uk/software/paml.html).