Our research is focussed primarily on speciation, in particular in reptiles. We use various model systems to analyse observed phylogeographic and population-genetic patterns, so as better to understand the regional genetic differentiation that occurs within species during the course of speciation. Insofar as possible, our investigations are designed to take account also of morphological and ecological parameters.
Many of our studies are concerned with widely distributed thermophilic western Palaearctic reptiles (e.g., Lenk et al. 1999; Fritz et al. 2005, 2007a, b, 2009; Böhme et al. 2007). The genetic structure of these species and their distribution ranges have been affected by Pleistocene extinctions, area fragmentation and Holocene area expansions. However, ongoing research projects are also concerned with various tropical species. Findings so far indicate that at least in some widespread species from the tropics a gradual genetic alteration associated with increasing geographical distance (Isolation by Distance) plays a greater role than it does in the western Palaearctic; an example is the widely distributed sub-Saharan leopard tortoise Stigmochelys pardalis (Fritz et al. 2010). Conversely, widespread habitat specialists everywhere, even in the tropics, exhibit genetic differentiation patterns reminiscent of those of Palaearctic reptiles (e.g., the red-footed tortoise Chelonoidis carbonaria in the Amazon region, a savannah species; Vargas-Ramírez et al. 2010), so that in such cases vicariance events may well be primarily responsible for the genetic structure. Below, we describe some of our current research projects.
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EUROPEAN POND TURTLE AND GREEN LIZARDS
A western Palaearctic species providing a model system that has been studied for many years is the European pond turtle (Emys orbicularis), which is distributed from North Africa over large parts of Europe, as far as Central Asia. More recently, phylogeographic and population-genetic studies on green lizards (Lacerta bilineata, L. viridis) have also been carried out; as thermophilic reptiles in Central Europe, these basically exhibit a similar Holocene colonization history, but with a different phylogeographic pattern. Whereas Central and Eastern Europe were recolonized by the pond turtle in two immigration waves from the east (Balkan and Black Sea region), northern green lizards are derived from an immigration wave from the west and another from the southeast. The genetic diversity of both pond turtles and green lizards is greatest in the south of the distribution region, where numerous genealogical lineages are distributed that survived the Pleistocene glacials there. Some of these lineages are endemic to small regions, more or less corresponding to their former glacial refuges. Each lineage represents a more or less advanced stage in the course of speciation. A highly differentiated lineage of the pond turtle is distributed in Sicily. In this case there has evidently been no gene flow with other pond turtle populations for a long time, which is why Sicilian pond turtles were described as a distinct new species (Emys trinacris; Fritz et al. 2005; Pedall et al. 2011). Green lizards are currently also assigned to two different species, Lacerta bilineata und L. viridis, even though a limited degree of gene flow still occurs (Rykena 1991). In this regard, our current investigations are designed to clarify the degree of gene flow and the status of a third lineage, which is distributed in the western Balkans and exhibits genetic differentiation comparable to Lacerta bilineata and L. viridis (Böhme et al. 2007).
Although turtles and tortoises are considered to be slow animals with limited dispersal abilities, the case of the E. orbicularisshows that turtles, too, can expand their ranges very rapidly. Originating from glacial refugia in the Balkan and the Black Sea region, pond turtles colonized a gigantic distribution area in the former permafrost region of Central and East Europe and Scandinavia during a brief period of the early Holocene. Our studies clarified in detail both the origin and the timing of this Holocene range expansion and the successive withdrawal process. After a rapid advance as far as southern Scandinavia and England (approx. 9800 years ago), the northernmost populations in Sweden had already vanished 5500 years ago, well before the Holocene thermal maximum ended in Scandinavia (Lenk et al. 1999; Fritz et al. 2007, 2009; Sommer et al. 2007, 2009). Today, just as in the case of the L. viridis, the northwesternmost relics of the pond turtle exist in Brandenburg, Germany. We are caretakers of these northern relic populations, in collaboration with the Brandenburg State Office for Environment (Dr. Norbert Schneeweiß), also in regard to population genetics.
Many species of land tortoises (Testudinidae) are characterized by a striking degree of morphological variability. Our investigations indicate that environmental factors strongly affect body size, coloration and shell shape. Consequently, morphological variability sometimes clearly conflicts with phylogeographic differentiation patterns (Fritz et al. 2005, 2007, 2010). In the case of the widespread Mediterranean spur-thighed tortoise (Testudo graeca) small-bodied, very light-coloured animals and substantially larger and darker specimens may occur in close proximity. Such tortoises are genetically not clearly differentiated. In this case the substrate coloration (lighter or darker background) and the humidity of the habitat (food availability!) seem to be responsible for the morphological differences (Fritz et al. 2007). The marginated tortoise (Testudo marginata) from the southern Balkan peninsula presents another example: Marginated tortoises from the hottest and driest part of the distribution area are genetically indistinguishable from considerably larger specimens from other regions (Fritz et al. 2005); the smaller individuals in this case were originally even considered to be a separate species. In the sub-Saharan leopard tortoise (Stigmochelys pardalis), again, there are considerable size differences that are not correlated with phylogeographic differentiation (Fritz et al. 2010; see also the figure showing two adult leopard tortoises), so that it can be inferred that external morphology of land tortoises is subject to very strong selective pressure, leading to a high degree of phenotypic plasticity.