Where ΔT signifies the transpose of Δ, and each equation from Section \ref{826836} is now additionally indexed according to population and has an additional term to account for migration between populations. While in principle the elements of Δ can take any value [0, 1], signifying a (continuous) movement of between 0 and 100% of individuals, for simplicity we use a constant value δ for the strength of each movement, i.e. for each non-zero off-diagonal element of Δ. Sensitivity to this value is explored in the supporting information (Fig \ref{175706}).