Contour plots of the infection rate of hybrids in copepods (A) and sticklebacks (B) that reproduce the observed data. Contours are plotted as a function of the infection rates of self-fertilized worms in copepods and the proportion of hybrid coracidia. See the main text for the equations used to calculate the hybrid infection rates. Black areas represent parameter space in which the observed results cannot be reproduced; intuitive explanations for this are given in each case. The gray areas in (B) are the black areas from (A). Dashed white lines delineate the parameter space that we consider most plausible. The boundaries of this area on the y-axis were based on observed hybridization rates, which ranged from 0.24 to 0.49. The width on the x-axis was based on the assumption that, due to inbreeding depression, the infection rate of selfers is probably lower than the overall mean (0.375).