Using genetic markers to directly estimate male selection gradients

We present an analysis of Raphanus raphanistrum and simulations illustratin g the utility of directly estimating male phenotypic selection gradients us ing genetic markers. The method offers a much more refined characterization of selection than attempting to assign paternity to individual progeny. Ou r analysis of R. raphanistrum reveals selection on remarkably fine features of floral morphology, including anther exsertion, that were opaque to prev ious approaches. The new results also undermine a previous conclusion that selection on wild radish floral morphology acts primarily through female fi tness. Simulation results show that selection gradients on the order of bet a = 0.1-0.2 can be readily detected with allozyme markers in moderate-sized (< 200 paternal individuals) populations. Highly polymorphic (e.g., micros atellite) markers will likely detect fine scale selection (<beta> < 0.1) in larger populations (<greater than or equal to> 400 individuals). Increased progeny sample size, by sampling either additional maternal families or mo re progeny per maternal parent, partly compensates for low exclusion probab ility. Increasing the number of possible fathers without changing progeny s ample size decreases the ability to detect selection, especially at lower e xclusion probabilities. Sampling only some male genotypes reduces the power to detect selection and biases (underestimates) the magnitude of the selec tion gradient estimate.