Which point mutation/substitution model will be used to modify
the sequences during the simulation? Options are "JC69" (Jules and Cantor
1969), "K80" (Kimura 1980), "F81" (Felsenstein 1981), "HKY85" (Hasegawa,
Kishino and Yano, 1985), "TN93" (Timura and Nei 1993), or "GTR" (General Time
Reversible Model, Tavar<U+00E9> 1986), note however that "GTR" is currently
unsupported.