 For a particular site, let
be the set of possible states for the site, and
their respective probabilities at time formula_20.For two distinct formula_21, let formula_22 be the transition rate from state formula_23 to state formula_24.Similarly, for any formula_23, let the total rate of change from formula_23 be

The changes in the probability distribution formula_28 for small increments of time formula_29 are given by

In other words, (in frequentist language), the frequency of formula_31's at time formula_32 is equal to the frequency at time formula_20 minus the frequency of the "lost" formula_31's plus the frequency of the "newly created" formula_31's.