This study examines the pattern of reaction norm variation and the ext
ent to which genetic constraints could prevent the evolution of broade
r ecological tolerance in Senecio integrifolius, a rare and declining
plant in Sweden. Following the establishment of biparental progenies,
we planted seedlings from each full-sib family in two greenhouse envir
onments: full daylight and 50 per cent shade. A 2-year demographic stu
dy indicated that full-sib genotypes representing a large population a
t a site with a fine-scale mosaic habitat (meadow with clumps of shrub
s) had a significantly greater overall performance across the environm
ents and a lower sensitivity to a drastic reduction in light intensity
than genotypes representing a small and patchily distributed populati
on at a site without spatial variation (meadow exposed to full sun), c
onfirming the supposition of more broadly adapted genotypes in a large
population subject to selection across a wider range of micro-habitat
s. There was a significant average effect of genotype for all traits b
ut we failed to detect within-population variation in phenotypic respo
nse (G x E) for direct components of fitness and conclude that there i
s little potential for specialization to particular light regimes. Acr
oss-environment correlations using full-sib means were small and posit
ive and there was no change in the ranking of population means across
the two environments, implying a lack of strong trade-offs within the
reaction norm. However, the changes in ecological amplitude may not be
of sufficient magnitude to permit long-term survival of remaining pop
ulations, as implied by the rapid decline of S. integrifolius in Swede
n.