Figure 1.

Schematic representation of the two habitat model. The top figure shows the growth rate (fitness) in each habitat as a function of the phenotypic trait z. In habitat i, the growth rate is assumed to be maximized at $z={\theta }_i,$ and the strength of selection is governed by $s_i$ (see Equation 1). Here, we illustrate a scenario with nonsymmetric fitness functions. The bottom figure shows the phenotypic density in each habitat (light blue and light red in habitats 1 and 2, respectively). Migration from population i is governed by the parameter $m_i,$ and tends to reduce the differentiation (i.e., the difference between the mean phenotypes) between populations.