Abstract
Level and partitioning of genetic diversity is expected to vary between contrasting habitats, reflecting differences in strength of ecological and evolutionary processes. Therefore, it is necessary to consider processes acting on different time scales when trying to explain diversity patterns in different parts of species' distributions. To explore how historical and contemporary factors jointly may influence patterns of genetic diversity and population differentiation, we compared genetic composition in the perennial herb Arabidopsis lyrata ssp. petraea from the northernmost parts of its distribution range on Iceland to that previously documented in Scandinavia. Leaf tissue and soil were sampled from ten Icelandic populations of A. lyrata. Seedlings were grown from soil samples, and tissue from above-ground and seed bank individuals were genotyped with 21 microsatellite markers. Seed bank density in Icelandic populations was low but not significantly different from that observed in Norwegian populations. While within-population genetic diversity was relatively high on Iceland (H E = 0.35), among-population differentiation was low (F ST = 0.10) compared to Norwegian and Swedish populations. Population differentiation was positively associated with geographical distance in both Iceland and Scandinavia, but the strength of this relationship varied between regions. Although topography and a larger distribution range may explain the higher differentiation between mountainous Norwegian relative to lowland populations in Sweden, these factors cannot explain the lower differentiation in Icelandic compared to Swedish populations. We propose that low genetic differentiation among Icelandic populations is not caused by differences in connectivity, but is rather due to large historical effective population sizes. Thus, rather than contemporary processes, historical factors such as survival of Icelandic lineages in northern refugia during the last glacial period may have contributed to the observed pattern.
Highlights
Genetic diversity and structuring of natural populations are shaped by both historical and contemporary processes within a given region [1]
In the present study we investigate whether patterns of genetic diversity within and among Icelandic populations of A. lyrata differ from that observed in Scandinavia
Since our results show no significant differences between Iceland and Scandinavia regarding seed bank characteristics, we have compared the above-ground statistics for various regions assuming that the differences observed for genetic parameters are likely not due to differences in seed dormancy patterns
Summary
Genetic diversity and structuring of natural populations are shaped by both historical and contemporary processes within a given region [1]. Much work has focused on the impact of local and regional habitat structure and landscape heterogeneity on levels of genetic diversity, differentiation, patterns of gene flow and adaptability of local populations [2,3,4,5]. Genetic connectivity between populations often varies across habitats [6] and different patterns of population differentiation may be expected among conspecific populations inhabiting regions in different parts of the distribution range [7]. Population history, including population age expressed as time since colonization, stability through time and historical effective sizes, may influence observed levels of structuring [12,13], and a diverse range of ecological and evolutionary processes must be examined in order to be able to explain patterns of biodiversity observed within species [14]. Extent of geographical barriers, dispersal ability and proximity to refugia may influence the association of population age on strength and patterns of IBD [14,20]
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