Figure 4

Fitness and its impact on the unstable equilibrium for complete Medea lethality ( d =1). The position of the internal unstable equilibrium is illustrated which needs to be traversed for population transformation and for reversal. As the value of ν increases the unstable equilibrium moves closer to the all ++ vertex. The different values of ω trace a curve which intersects the Hardy-Weinberg equilibrium line at ν=ω. For underdominance the fixed points are always below the Hardy-Weinberg curve (also see Figure 7). This also graphically demonstrates Eqs. (A.4) and (A.5) i.e. the frequency of the Medea allele is 1/2 when ν+ω=1 (vertical line, which also represents the ideal with respects to the ease of transformation and its reversal, see Figure 3). Note that when the unstable equilibrium is above the Hardy-Weinberg equilibrium line, there also exists a stable root on the M+ – MM edge given by $(\widehat{x},ŷ)=(\frac{\nu }{2\omega -\nu },1-\widehat{x})$. Disks indicate the positions of results plotted in Figure 2 for the ‘Medea only’ system (M) and the combined system (C).