Environmental effects on genetic variance are likely to constrain adaptation in novel environments

Adaptive plasticity allows populations to cope with environmental
variation but is expected to fail as conditions become unfamiliar. In
novel conditions, populations may instead rely on rapid adaptation to
increase fitness and avoid extinction. Adaptation should be fastest when
both plasticity and selection occur in directions of the multivariate
phenotype that contain abundant genetic variation. However, tests of this
prediction from field experiments are rare. Here, we quantify how additive
genetic variance in a multivariate phenotype changes across an elevational
gradient, and test whether plasticity and selection align with genetic
variation. We do so using two closely related, but ecologically distinct,
sister species of Sicilian daisy (Senecio, Asteraceae) adapted to high and
low elevations on Mount Etna. Using a paternal half-sibling breeding
design, we generated and then reciprocally planted c.19,000 seeds of both
species, across an elevational gradient spanning each species’ native
elevation, and then quantified mortality and five leaf traits of emergent
seedlings. We found that genetic variance in leaf traits changed more
across elevations than between species. The high-elevation species at
novel lower elevations showed changes in the distribution of genetic
variance among the leaf traits, which reduced the amount of genetic
variance in the directions of selection and the native phenotype. By
contrast, the low-elevation species mainly showed changes in the amount of
genetic variance at the novel high elevation, and genetic variance was
concentrated in the direction of the native phenotype. For both species,
leaf trait plasticity across elevations was in a direction of the
multivariate phenotype that contained a moderate amount of genetic
variance. Together, these data suggest that where plasticity is adaptive,
selection on genetic variance for an initially plastic response could
promote adaptation. However, large environmental effects on genetic
variance are likely to reduce adaptive potential in novel environments.