PAML: a program package for phylogenetic analysis by maximum likelihood.

PAML, currently in version 1.2, is a package of programs for phylogenetic analyses of DNA and protein sequences using the method of maximum likelihood (ML). The programs can be used for (i) maximum likelihood estimation of evolutionary parameters such as branch lengths in a phylogenetic tree, the transition/transversion rate ratio, the shape parameter of the gamma distribution for variable evolutionary rates at sites, and rate parameters for different genes; (ii) likelihood ratio test of hypotheses concerning sequence evolution, such as rate constancy and independence among sites and rate constancy among lineages (the molecular clock); (iii) calculation of substitution rates at sites and reconstruction of ancestral nucleotide or amino acid sequences; and (iv) phylogenetic tree reconstruction by maximum likelihood and Bayesian methods. The strength of PAML, in comparison with other phylogenetic packages currently available, is its implementation of a variety of evolutionary models. These include several models of variable evolutionary rates among sites, models for combined analyses of multiple gene sequence data and models for amino acid sequences. Multifurcating trees are supported, as well as trees in which some sequences are ancestral to some others. A heuristic tree search algorithm (star decomposition) is used in the package, but tree making is not a strong point of the current version, although work is under way to implement efficient search algorithms. Major programs in the package, as well as the types of analyses they perform, are listed in Table 1. More details are available in the documentation included in the package, written using Microsoft Word. PAML is distributed free of charge for academic use only. The package, including ANSI C source codes, documentation, example data sets, and control files, can be obtained by anonymous ftp at mw511.biol.berkeley.edu/pub, or from the Indiana molecular biology ftp site at ftp.bio.indiana.edu under the directory Incoming or molbio/evolve . MAC and PowerMac executables are also available, although DOS executables are not prepared yet. Further information about the package is available from the World Wide Web at