Ecological amplitude in Silene nutans in relation to allozyme variation atthe western margin of its distribution

At the western border of its geographical range, Silene nutans (Caryophylla ceae) has evolved two groups of parapatric populations showing distinct all ozyme patterns and apparently occurring on different bedrock types. This st udy tests the hypothesis that these groups represent edaphic ecotypes. With this in view, the ecological amplitude of 36 populations of Silene nutans from Belgium was investigated and their synecology specified using vegetati on composition and soil parameters; Ellenberg indicator values were used fo r ecological interpretation of the vegetation analyses. The results provide evidence that allozyme and habitat variations are correlated, the two grou ps occurring on contrasting soil with distinct vegetation types. One group is restricted to alkaline soils and typically occurs in open calcareous gra sslands, fringes and scrub. The other group is characterized by wider autec ological and synecological ranges with a bimodal pH-distribution, occurring on two kinds of siliceous soils: (1) neutral soils, supporting vegetation with a high species richness comprising many mesophilous species, and (2) C aCO3-free bedrocks characterized by acidophilous, species-poor, open dry gr asslands and woodlands. It also appears that factors not related to soil ch emistry, such as microclimate, are involved in the ecological specializatio n of the two ecotypes. It is argued that these ecotypes exemplify adaptive radiation and parapatric speciation at the margin of a species distribution area.