Conflicting selection pressures on seed size: evolutionary ecology of fruit size in a bird-dispersed tree, Olea...

Recent evidence indicates that fruit size has evolved according to dispersers' size. This is hypothesized to result from a balance between factors favouring large seeds and dispersers setting the maximum fruit size. This hypothesis assumes that (1) the size of fruits that can be consumed by dispersers is limited, (2) fruit and seed size are positively correlated, and (3) the result of multiple selection pressures on seed size is positive. Our studies on the seed dispersal mutualism of Olea europaea have supported the first and second assumptions, but valid tests of the third assumption are still lacking. Here we confirm the third assumption. Using multiplicative fitness components, we show that conflicting selection pressures on seed size during and after dispersal reverse the negative pattern of selection exerted by dispersers.