There is general agreement that we need a better understanding of how historical processes contribute to patterns of variation in phenotypic plasticity within and among species; however, the evolution of reaction norms has rarely been addressed from within an explicitly phylogenetic comparative framework. Furthermore, the simultaneous evolution of traits in response to environmental variation characterized by different grains (coarse and fine from the standpoint of the organism being considered) is also amenable to phylogenetic comparative studies which have not been attempted so far. In this paper we compare the reaction norms to foliage shade (changes in light quality, spatially fine-grained environmental variation) and photoperiod (daylength, spatially coarse-grained environmental variation) in several haplotypes of Arabidopsis thaliana and of two closely related species, A. arenosa and A. lyrata subsp. petraea, from Scandinavia. We found that across-environment means evolved continuously and very rapidly within this group, while plasticity changed only rarely and especially between the outgroups and A. thaliana. Character means evolved largely independently of each other, while trait plasticities were highly integrated, as predicted by the adaptive plasticity hypothesis for response to foliage shade (the so-called “shade avoidance” syndrome). We found evidence of strong constraints across environmental factors (daylength and foliar shade) for some traits directly related to life history, but otherwise largely independent evolution of the reaction norms of many traits in response to either daylength or light quality. Some of the observed patterns can be explained by a combination of shared ecological circumstances and an underlying genetic constraint due to the fact that an overlapping battery of photoreceptors perceive both aspects of light availability.

Key words: Arabidopsis - phenotypic plasticity - light availability - ecotypes - comparative method