An important basis of modern phylogenetics describing the mathematical basis of mutation

In phylogenetics this describes the frequency of A, G, T and C mutations as a 2 x 2 matrix which is either reversible A <-> C occurs at the same frequency as C <-> A or directional where the two frequencies are independent. Common models include Kimura's 2 parameter model combining an estimate of reversion mutation with the frequency of pyrimidine to pyrimidine (or purine to purine) mutations in comparison to pyrimidine to purine or purine to pyrimidine mutations, denoted as transitions and transversions respectively.