Dispersal is a central process to almost all species on earth, as it connects spatially structured populations and thereby increases population persistence. Dispersal is subject to (rapid) evolution and local patch extinctions are an important selective force in this context. In contrast to the randomly distributed local extinctions considered in most theoretical studies, habitat fragmentation or other anthropogenic interventions will lead to spatially correlated extinction patterns. Under such conditions natural selection is thought to lead to more long-distance dispersal, but this theoretical prediction has not yet been verified empirically. We test this prediction in experimental spatially structured populations of the spider mite Tetranychus urticae and supplement these empirical results with insights from an individual-based evolutionary model. We demonstrate that the spatial correlation of local extinctions changes the entire distribution of dispersal distances (dispersal kernel) and selects for overall less emigration but more long-distance dispersal.

Debates on speciation processes in pteridophytes have revived. In order to study the evolutionary origin of an apomictic fern species, we investigated the genetic variation in the strictly agamosporous Dryopteris remota. We determined the genotypes of 22 individuals from many different locations within the species' European distribution and of 20 individuals from a Swiss population. A previous study on isozyme variation showed no intraspecific genetic variation in a similar sample set (Schneller and Holderegger, 1994, American Fern Journal 84: 94-98). In contrast to this, four out of 12 random amplified polymorphic DNA (RAPD) primers tested revealed low genetic diversity among individuals of D. remota from different locations. Intrapopulational genetic variation was also very low, but in the single population studied, a unique multiband genotype could be detected. The geographic distribution of genetic variation found in D. remota was best explained by the assumption of a single origin, the accumulation of somatic mutations during spread, and occasional, but effective, events of dispersal over large distances. The present study thus stresses the importance of long-distance dispersal in evolutionary processes and biogeography of ferns.

Several alpine species have outlying populations in the lowlands and lower mountains north of the Alps. These small, isolated populations are usually described as either (1) glacial relics, (2) descendants from populations living on forelands and moraines during the ice ages, or (3) populations founded by long-distance dispersal after glaciation. A floristic survey of the historic and present distributions and an allozyme investigation were performed on one of these relic species, Saxifraga aizoides. The species was historically more abundant and had more stations in more regions of northeastern Switzerland. The former population structures within regions, nowadays destroyed, were still reflected in distinct and high regional genetic diversity and variation. There was weak evidence of increased inbreeding in outlying populations, but populations did not deviate from Hardy-Weinberg equilibrium. No geographic pattern of genetic variation above the regional scale (Ͼ10 km) was found. Based on the spatial and genetic structures found, it was not possible to discriminate between the abovementioned hypotheses. Nevertheless, the study shows how a thorough evaluation of distribution and abundance data aids the interpretation of genetic data with respect to population history, biogeography, and conservation biology. Key words:allozymes; Alps; biogeography; conservation biology; floristic survey; glacial relics; long-distance dispersal; Saxifraga aizoides; Saxifragaceae.Past events play an important role in the evolution of species and populations. Plant biogeography and phylogeography are especially informative of how the ice ages have shaped the genetic structure of species (Soltis et al., 1997). Partial extinctions, recurrent (re-)immigration, different migration routes, long-distance dispersal events, bottleneck situations, in situ survival in refugia, or gentic drift are crucial phenomena in this context. In Europe, most molecular studies have so far focused on arcticalpine plants or on tree species (Comes and Kadereit, 1998). Available results on arctic-alpine plants are contradictionary, giving either evidence in favor of glacial refugia or against them (Abbott et al., 1995;Brochmann et al., 1996;Gabrielsen et al., 1997; Bauert et al., 1998). Smouse (1998) stressed that contradictions between different studies even on the same organism may result from inaccurate sampling strategies or inappropriate choice of genetic markers to test particular biogeographic hypotheses.In the mountainous Prealps and the hilly adjacent lowlands north of the Alps, several subalpine or alpine plant 1 Manuscript received 19 January 1999; revision accepted 23 July 1999.The authors thank Andreas Keel, Elias Landolt, and Hansruedi Wildermuth for providing present stations of Saxifraga aizoides, and Richard J. Abbott, Alex Bernhard, Robert M. M. Crawford, and two anonymous reviewers for critical comments on the manuscript. A grant for laboratory consumables by the Ü etliberg-Verein to JJS and RH and financial support by the Swiss National Sci...