Selection maintains a nonadaptive floral polyphenism

Adaptive phenotypic plasticity evolves in response to the contrasting selection pressures that arise when organisms face environmental heterogeneity. Despite its importance for understanding how organisms successfully cope with environmental change, adaptive plasticity is often assumed but rarely demonstrated. We study here the adaptive nature of the extreme seasonal within-individual floral polyphenism exhibited by the crucifer Moricandia arvensis, a Mediterranean species that produces two different types of flowers depending on the season of the year. During spring, this species has large, cross-shaped, lilac flowers, while during summer, it develops small, rounded, white flowers. Although floral polyphenism was associated with increased plant fitness, selection moved floral traits away from their local optimum values during the harsh summer. This result strongly suggests that floral polyphenism is not adaptive in M. arvensis. The main factor selecting against floral polyphenism was pollinators, as they select for the same floral morph in all environments. Despite not being adaptive, floral polyphenism occurs throughout the entire distribution range of M. arvensis and has probably been present since the origin of the species. To solve this paradox, we explored the factors causing floral polyphenism, finding that floral polyphenism was triggered by summer flowering. Summer flowering was beneficial because it led to extra seed production and was favored by adaptive plasticity in leaf functional traits. Taken together, our study reveals a complex scenario in which nonadaptive floral polyphenism has been indirectly maintained over M. arvensis evolutionary history by selection operating to favor summer flowering. Our study provides thus strong evidence that nonadaptive plasticity may evolve as a byproduct of colonizing stressful environments. How organisms adapt to changing environments is a central question in ecology and evolution. Adaptive plasticity is a pervasive strategy used by organisms to successfully cope with environmental change. Here we study the adaptive nature of a within-individual floral polyphenism widely displayed by a Mediterranean weed. During spring, this species produces large, cross-shaped, lilac flowers, while during summer, it develops small, rounded, white flowers. Despite being associated with increased fitness, this floral plasticity was not adaptive because pollinators consistently prefer the same floral phenotype across environments and select against floral plasticity. The prevalence of this nonadaptive floral plasticity is an indirect consequence of plants benefiting from flowering under stressful conditions that prevent the expression of optimal floral phenotypes. We provide strong evidence that nonadaptive plasticity may evolve when organisms colonize and establish in stressful conditions. Moricandia arvensis is a Mediterranean weed that produces two different types of flowers depending on the season of the year. During spring, the individuals of this species have large, cross-shaped, lilac flowers, while during summer, the same individuals produce small, rounded, white flowers. We explored whether this polyphenism is the result of natural selection, making the plant better adapted to the dominant pollinators each season. Our integrative study has found out that, despite its morphological complexity and uniqueness, M. arvensis floral polymorphism is not adaptive because pollinators select the same floral morph in all environments. Although not adaptive, floral polyphenism occurs throughout the entire distribution range of M. arvensis and has probably been present since the origin of the species. To solve this dilemma, we explored the ecological mechanisms causing floral polyphenism and found that floral polyphenism was a consequence of flowering during summer, a season with extreme weather conditions in the Mediterranean. Our study provides thus strong evidence that nonadaptive plasticity may evolve as a byproduct of colonizing stressful environments.