SSA was developed to be a program for inferring maximum likelihood phylogenies from DNA sequences. Two versions of the program are available: one which assumes a molecular clock and one which does not make this assumption. The method for searching the space of trees for the ML tree is based on a simulated-annealing type algorithm and is described in the reference above. The program implements Felsenstein's F84 model of nucleotide substitution and associated sub-models. The program estimates the ML tree and branch lengths, and can optionally estimate the transversion / transversion ratio. Upon termination, the program returns the k trees of highest likelihood found during the search, where k can be set by the user.