A stable pollination environment limits current but not potential evolution of floral traits

Plants9 vast variation in floral traits at a macroevolutionary level is often interpreted as the result of adaptations to pollinators. However, field studies measuring pollinator-mediated evolution of flowers often find no evidence for directional selection. A possible explanation is the prevalence of periods of stasis, when selection on flowers is relaxed under stable pollination conditions, followed by unstable periods where pollinator changes provide innovative selection. Here we asked if periods of stasis are the consequence of stabilizing or no directional selection on traits, or of low levels of heritable variation even if selection is present. We measured heritability and evolvability of floral traits, using genome-wide molecular relatedness of wild plants, combined with estimates of selection on the same individuals to estimate evolutionary potential. We studied Ulex parviflorus, a plant predominantly pollinated by a single bee species across its range. We found evidence for both stabilizing selection and low trait heritability as explanations for stasis in flowers. The area of the standard petal is currently under stabilizing selection, but the variability we observe in the wild is not heritable. Floral size in turn presents high field heritability, but is not currently under selection. We provide an example of a stable environment that has led to a lack of directional selection, yet maintaining enough heritable variation for responding to possible novel selection pressures.