  Organisms modulate their fitness in heterogeneous environments by dispersing.
Prior work shows that there is selection against "unconditional" dispersal in
spatially heterogeneous environments. "Unconditional" means individuals
disperse at a rate independent of their location. We prove that if within-patch
fitness varies spatially and between two values temporally, then there is
selection for unconditional dispersal: any evolutionarily stable strategy (ESS)
or evolutionarily stable coalition (ESC) includes a dispersive phenotype.
Moreover, at this ESS or ESC, there is at least one sink patch (i.e. geometric
mean of fitness less than one) and no sources patches (i.e. geometric mean of
fitness greater than one). These results coupled with simulations suggest that
spatial-temporal heterogeneity due to abiotic forcing result in either an ESS
with a dispersive phenotype or an ESC with sedentary and dispersive phenotypes.
In contrast, spatial-temporal heterogeneity due to biotic interactions can
select for higher dispersal rates that ultimately spatially synchronize
population dynamics.
