A paternal half-sib analysis was performed to estimate the narrow-sens
e heritability for 35 morphological and life-history characters in two
populations of Senecio integrifolius, a rare and threatened plant in
Sweden. The two populations differed greatly in size, the small one co
nsisting of separate patches and the large one having a more or less c
ontinuous distribution. In addition to significant population differen
ces in the mean of 26 characters, we found slightly higher heritabilit
ies in the small and patchy population (average 0.49) than in the larg
e and continuous population (average 0.43). Overall, the small populat
ion displayed significant additive genetic variation for a larger numb
er of characters than the large population (19 vs. 14). Paternal (gene
tic) effects were consistent in two different environments; the rankin
g of sire means was similar despite a drastic reduction in light inten
sity, and few characters had a significant sire x environment interact
ion. Genetic correlations were also found between different characters
measured in the same environment, although most of these were in the
'favourable' direction. Population differences in genetic parameters m
ay reflect contrasting selection regimens at the two sites, although g
enetic drift may also be important. That more genetic variability was
detected in the small population where plants occur in small and isola
ted patches indicates that the spatial structure has great effects on
the retention of heritable variation, and that demographic and environ
mental factors are more important in the long-term survival of S. inte
grifolius. This is the first study that relates quantitative genetic v
ariation to population size in a rare and threatened plant.