THE STRUCTURE OF ALLOZYME AND LEAF SHAPE VARIATION IN ISOLATED, RANGE-MARGIN POPULATIONS OF THE SHRUB HIPPOCREPIS-EMERUS (LEGUMINOSAE)

We investigated allozyme variation and phenotypic variation in leaf shape in 15 populations of the shrub Hippocrepis emerus (Leguminosae) from the three isolated, regional populations in Scandinavia and analysed patterns of differentiation and the hierarchical structuring of diversity on different geographic scales. There are pronounced geographic differences between the Norwegian and Swedish isolates of the species and most of the polymorphic allozyme loci show reciprocal fixation in the two isolates. The Scandinavian populations of H, emerus rue not only disjunct on a macrogeographic scale but also show considerable disjunction within regions. Within the Gland regional population? a central group of populations shows low levels of inter-population differentiation: the Gotland group of populations is related to this core group of Gland populations. The geographically marginal populations on Oland are spatially isolated and show a higher degree of divergence between populations than does the central group of Oland populations. We interpret genetic divergence between the marginal populations in terms of generic drift - as a result of historical fluctuations in habitat availability and population size.