Introduction

The Evolutionary Synthesis of Dobzhansky, Mayr, Rensch, Wright, Simpson, and others communicated the idea of population genetics to other fields of biology. These authors argued convincingly that mutation, recombination, selection, and gene flow operating within species ("microevolution" in Dobzhansky's term) account for the origin of new species and for the long-term effects of evolution ("macroevolution" in Dobzhansky's term) (Dobzhansky 1937). The European exponent of Evolutionary Synthesis, Bernhard Rensch (Futuyma 2005), was very interested in the processes of genetic drift as can be studied easily in small populations, such as on islands and in glacial refuges (Rensch 1929, 1939, 1951). A lively discussion on the processes of differentiation, leading to racial formation and spe-ciation, was held in these years (Rensch 1929, 1939, 1960; Krumbiegel 1932, 1936; De Lattin 1959). While Krumbiegel (1936) described and measured morphological differences as a result of these processes, the genetic consequences of the climate-induced range shifts, including postglacial recolonization, were hypothesized for the first time by Reinig (1938, 1939). Recently, this idea was confirmed by compilations of studies using molecular data (Hewitt 1996, 1999; recent review by Schmitt 2007).

Biogeography profited from the development of both evolution-orientated population biology and the classical models of biogeography by pioneers, such as Holdhaus (Holdhaus and Lindroth 1939) and De Lattin (1957, 1959, 1967). Holdhaus showed that the distribution of blind euedaphic and troglobitic (cave-inhabiting) beetles is restricted to a well-defined area in Europe (Holdhaus 1954). In the South of the "Holdhaus line", defined by the Northern distribution limit of these species, many endemic species occur. These areas, which coincide with the occurrence areas of those for endemic plant species of the Alps, have been known as "Massifs de Refuge" after Chodat and Pampanini (1902), and were discussed by Holdhaus (1906) as refuge areas also suitable for animals.

Surprisingly, Holdhaus' ideas on glacial refuges and postglacial colonization processes have never been rejected although contemporary Anglophone scientific work and biogeography primers have mostly ignored them. One reason for this might be that these basal works have mainly been published in German.

Over the last decades, our knowledge of processes of glacial and postglacial periods has increased enormously, mainly due to the progress in evolutionary genetics and the use of a variety of genetic markers (e.g., Stauffer et al. 1999; Seddon et al. 2001; Michaux et al. 2003; Berggren et al. 2005; Joger et al. 2007) and the combination of these results with other methods (e.g., analysis of pollen and fossil records) to reconstruct the past events (Terhurne-Berson et al. 2004; Cheddadi et al. 2006; Magri et al. 2006; Magri 2008). Interestingly, not only historical but also recent hypotheses and postglacial recolonization scenarios are frequently based on studies of insect species, especially on those with low dispersal power (e.g., Hewitt 1996).

The aims of our study are to test whether

1. the Holdhaus line is congruent with the current knowledge on the distribution of endemics in Europe. We use ground beetles as a taxonomic group for this reevaluation as they are well studied in Europe at most relevant levels (especially faunistics and taxonomy, Assmann et al. 2008). This beetle family comprises a vast number of species in Europe (over 3,600), many of which are blind. Thus, this taxon also seems appropriate to test the validity of Holdhaus' biogeographic description of the Northern limit of blind beetles.

2. the Holdhaus line can be used for analytical procedures in present-day phylogeog-raphy of European species which aim to distinguish between relict and (re-)coloniz-ing species (and populations), and to delimit glacial refuges. We use the stenotopic flightless woodland ground beetle species Carabus auronitens as a model species.

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