The microspatial genetic structure of allele frequencies at seven isozyme l
oci was examined for 15 populations of the land snail Helix aspersa sa samp
led in a village from Brittany (north-western France). Spatial heterogeneit
y of allele frequencies was highly significant (P < 0.001). Fixation indice
s reflected nonrandom mating within neighbourhoods and a slight but consist
ent differentiation between colonies (F-ST = 0.044; P < 0.01). Analyses of
gene flow or genetic distances failed to reveal a significant relationship
with geographical distance, probably because of the complexity of environme
ntal heterogeneity. However, matrix comparisons between genetic distances a
nd connectivity networks among adjacent colonies (Gabriel-connected graph)
yielded a significant correlation in every case, indicating a 'step-by-step
' relationship between neighbouring localities. Moreover, most of the alloz
ymes were spatially structured and showed (i) a gradual isolation of coloni
es with increasing geographical distances, and (ii), for some correlogram p
rofiles, a circular gradient illustrating a multidirectional colonization o
f the village. The probable existence of disperser individuals allowed us t
o suggest a metapopulation model which would explain the maintenance of suc
h animals in fragmented habitats where anthropogenic disturbances and extin
ction/recolonization events are commonly observed.