The effect of unequal migration rates on FST

Using the structured coalescent model, it is shown that unequal migration rates between different pairs of subpopulations can increase the value of Wrightʹs coefficient FST and its dependence on the mutation rate, and decrease the effective level of gene flow. Two specific models of population structure are considered: (i) an island model with barrierʹ where migration rates between subpopulations on the same side of the barrier are higher than migration rates between subpopulations on opposite sides of the barrier, and (ii) the two-dimensional stepping-stone model with unequal migration rates in the two dimensions of the model.