Sweet, but not too sweet Nectar-feeding pollinators, when given a choice, tend to prefer nectar with high concentrations of sugar. Nectar-producing plants, however, tend to produce more dilute nectar. This mismatch between selective force and trait value has long been seen as an evolutionary paradox. Nachev et al. used a dynamic flower array that evolved in real time in a Costa Rican rainforest to show that the “paradox” is, in fact, driven by pollinator choices (see the Perspective by Farris). Bat pollinators based their choices on small, nonlinear differences in nectar sweetness, which led to selection for less sweet nectar overall. Science , this issue p. 75 ; see also p. 25