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

Plants’ vast variation in floral traits at a macroevolutionary level is often interpreted as the result of adaptation to pollinators. However, field studies often find no evidence of pollinator-mediated selection on flowers. This could be explained by periods of stasis, when selection is relaxed under stable conditions, followed by pollinator changes that provide innovative selection. We asked if periods of stasis are caused by stabilizing or absence of other forms of selection on floral traits, or by low trait heritability even if selection is present. We studied Ulex parviflorus, a plant predominantly pollinated by one bee species across its range. We measured heritability and evolvability of floral traits, using genome-wide molecular relatedness in a wild population, and combined this with estimates of selection. We found evidence for both stabilizing selection and low trait heritability as explanations for stasis in flowers. The area of the standard petal is under stabilizing selection, but the variability observed in the wild is not heritable. A separate trait, floral size, in turn presents high heritability, but is not currently under selection. We show how a stable pollination environment can lead to a lack of evolutionary change, yet maintain heritable variation to respond to future selection pressures.

A phylogenetic analysis of trait convergence in the spring flora1This article is part of a Special Issue entitled “Pollination biology research in Canada: Perspectives on a mutualism at different scales”.

In temperate deciduous forests, spring flowering plants exhibit remarkable similarity in a number of characteristics, including reproductive, vegetative, and ecological traits. The apparent convergence of floral traits, especially corolla colour, among spring flowering species has been well documented, but remains poorly understood. Here we review adaptive hypotheses and predictions that have been proposed to explain the apparent correlation between spring flowering and a suite of traits. We investigated the correlation between flowering phenology (i.e., spring or nonspring) and several key traits using phylogenetic comparative methods. Through this analysis we were able to confirm the existence of a correlation for five of the six traits examined. Specifically, spring flowering is shown to have evolved in a correlated fashion with reproductive schedule (perennial vs. annual), light corolla colour, fruit type, growth form, and forest strata layer. In general, our survey determined that spring flowering species are perennial, have light coloured corollas, a herbaceous growth form, and tend to occupy the understory of the forest. These results are discussed in light of the reviewed adaptive hypotheses and the spring pollination environment.

Adaptive plasticity of floral display size in animal-pollinated plants

Plants need not participate passively in their own mating, despite their immobility and reliance on pollen vectors. Instead, plants may respond to their recent pollination experience by adjusting the number of flowers that they display simultaneously. Such responsiveness could arise from the dependence of floral display size on the longevity of individual flowers, which varies with pollination rate in many plant species. By hand-pollinating some inflorescences, but not others, we demonstrate plasticity in display size of the orchid Satyrium longicauda . Pollination induced flower wilting, but did not affect the opening of new flowers, so that within a few days pollinated inflorescences displayed fewer flowers than unpollinated inflorescences. During subsequent exposure to intensive natural pollination, pollen removal and receipt increased proportionally with increasing display size, whereas pollen-removal failure and self-pollination accelerated. Such benefit–cost relations allow plants that adjust display size in response to the prevailing pollination rate to increase their attractiveness when pollinators are rare (large displays), or to limit mating costs when pollinators are abundant (small displays). Seen from this perspective, pollination-induced flower wilting serves the entire plant by allowing it to display the number of flowers that is appropriate for the current pollination environment.