Variation in the genetic architecture of ecologically relevant traits along elevational gradients

Species are confined within spatial boundaries that are likely shaped by multiple factors. One understudied factor is the genetic non-independence of traits, which can constrain the adaptive potential. We tested this hypothesis on elevational gradients, where environmental conditions change over short geographic distances. We collected seeds of three montane and three alpine Brassicaceae species, each from its core and both the lower and upper range edges, and reared offspring plants of matrilines in the greenhouse under two thermal treatments: warm and control. We measured key growth and leaf traits known to vary with elevation in Brassicaceae and estimated the genetic covariance (G-)matrix for each population under each growth treatment. Genetic variances were reduced in lower-edge populations of montane species, whereas those of alpine species did not differ over the elevational gradients but were consistently low. Genetic integration-the reduction in evolvability due to genetic covariances-varied little over the elevational gradients and was generally high. In line, conditional multivariate evolvability was largely consistent and low across populations. Finally, populations had substantial genetic variation in the direction of population divergence under control conditions but not under warm conditions. Overall, our findings suggest that considerable genetic integration restricts evolution along certain trait axes, hindering niche expansion and thereby contributing to range boundaries.