  Understanding the influence of an environment on the evolution of its
resident population is a major challenge in evolutionary biology. Great
progress has been made in homogeneous population structures while heterogeneous
structures have received relatively less attention. Here we present a
structured population model where different individuals are best suited to
different regions of their environment. The underlying structure is a graph:
individuals occupy vertices, which are connected by edges. If an individual is
suited for their vertex, they receive an increase in fecundity. This framework
allows attention to be restricted to the spatial arrangement of suitable
habitat. We prove some basic properties of this model and find some
counter-intuitive results. Notably, 1) the arrangement of suitable sites is as
important as their proportion, and, 2) decreasing the proportion of suitable
sites may result in a decrease in the fixation time of an allele.
