Longitudinal patterns in species richness and genetic diversity in European oaks and oak gallwasps

While latitudinal patterns of genetic diversity are well known for many taxa in Europe, there has been little analysis of longitudinal patterns across Pleistocene glacial refugia. Here we analyze longitudinal patterns in two aspects of diversity (species richness and intraspecific genetic diversity) for two trophically related groups of organisms - oaks (Fagaceae, genus Quercus) and their associated gallwasps (Hymenoptera: Cynipidae) - across four southern refugial regions (Asia Minor, the Balkans, Italy, and the Iberian Peninsula). Two major patterns emerge: a) both intraspecific genetic diversity and species richness are lower in Iberia than in eastern refugia; and b) though depauperate, Iberia contains substantial endemic diversity both in terms of intraspecific variation and species. Sequence data for two of three gallwasp case studies show that Iberian lineages diverged from central and eastern European lineages well before the Pleistocene. However, coalescence times for Iberian lineages are far more recent, suggesting that Iberian populations may have a recent history of genetic bottlenecks. We suggest that these patterns are the result of two processes: a) the formation of distinct refugial biotas towards the end of the Pliocene, and b) longitudinal variation in the magnitude of climatic fluctuations during the Pleistocene. Specifically, we suggest that the lower richness in Iberia is the result of a recent history of stronger fluctuations and much more intense aridity than areas further east. Data for equivalent analyses in other taxa exist. Comparison of patterns across taxa should allow the genesis and testing of hypotheses for the origin and maintenance of longitudinal patterns of biodiversity in the Western Palaearctic.