An example of our general two-stage mutation process used in this article is as follows. The regulatory region contains two possible binding sites, a and b, where a prime denotes an inactivated site. Wild-type individuals can undergo a type
A point mutation (a b′
a′ b′ at rate
u1), which inactivates site a, and type
A individuals can undergo a type
B point mutation (a′ b′
a′ b at rate
u2), which creates a new active site b. The relative fitnesses of wild type,
A mutant, and
B mutant are 1,
r, and
s, respectively. Note that in this case, wild-type individuals cannot produce individuals with a second active binding site. For a different example of this general process, see p. 955 of Carter and Wagner (2002).