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

Abstract

 Plant´s exquisite 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 little evidence for strong 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 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 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.

Rapid evolution of a floral trait following acquisition of novel pollinators

“…; reviewed in Parachnowitsch et al, 2019) and are thus likely to require long-term consistent selection for a detectable response. This is in contrast to linear morphological traits that often present high values of heritability, even when measured in field conditions (Ashman & Majetic, 2006;Castellanos et al, 2019), and that have been shown to change in response to single mutations with implications for pollinator visitation (see Ding et al, 2017 for an example in Mimulus). For D. purpurea we are in the process of measuring heritability both in the field (using molecular markers) and in a common garden, and preliminary results from the field studies point towards very high and significant narrow sense heritabilities (h 2 > 0.45) for linear corolla traits (Castellanos M.C., unpubl.).…”

Human-Mediated Land Use Change Drives Intraspecific Plant Trait Variation

“…In a study that compared floral traits across time from a site with known pollinator declines, contemporary populations had larger floral displays and longer receptivity compared to their ancestor population ( Thomann et al, 2015 ). Similarly, Castellanos et al (2019) demonstrate that stable pollinator environments can stifle current evolutionary change while maintaining heritable variation necessary for future adaptation.…”

Quantitative genetic analysis of floral traits shows current limits but potential evolution in the wild

“…Datasets and R code for all analyses are available in the Dryad Digital Repository [ 74 ]. Supplementary methods, figures and tables are provided in the electronic supplementary material [ 75 ]. Raw sequence data can be accessed from the NCBI Sequence Read Archive (SRA) with BioProject accession PRJNA951080.…”