Silene dioica (Caryophyllaceae) has evolved different morphotypes that
vary in hairiness in different habitats in the Swedish mountains. A h
airy form, var. serpertinicola and a hairless (glabrous) form, var, sm
ithii grow on serpentine, i.e. a dry soil rich in heavy metals. Var. l
apponica is a densely hairy morphotype on subalpine meadows. Crosses b
etween these varieties showed that glabrousness has a simple Mendelian
inheritance and that glabrousness is determined by a recessive allele
. The crosses further suggested that the recessive allele has been pre
sent in the original population that invaded the mountain area. Glabro
us plants have, however, never been found on subalpine meadows, even t
hough an earlier study has shown that gene flow is relatively common a
mong serpentine and nearby meadow populations. In addition to the gene
tic background of glabrousness, we studied the grazing on glabrous smi
thii and hairy serpentinicola and Iapponica plants by the slugs Arion
fasciatus and Arion subfuscus and the snail Arianta arbustorum in gree
nhouse and in nature. Arion slugs, abundant on subalpine meadows, pref
erred glabrous plants and, in particular, A. fasciatus consumed high a
mounts of S. dioica. Arianta snails, however, consumed irrespective of
morphotype. The selective behaviour was seen both at the seedling sta
ge and the adult plant stage. Arion slugs were more choosy when they w
ere given a choice of glabrous smithii and densely hairy lapponica pla
nts then when they were served smithii and the less hairy variety serp
entinicola. This study implies that the Arion slugs are important agen
ts of selection on subalpine meadows. The selective grazing seems to c
ounteract the flow of the recessive allele that determines glabrousnes
s, from serpentine into meadow populations. Gastropods are rare in the
dry serpentine habitat. The evolution of a distinct glabrous morphoty
pe on serpentine is therefore rather due to relaxation of selection th
an to adaptation to serpentine stress.