Glacial Refugia and Postglacial Recolonization Patterns of Organisms

Glaciations especially the Pleistocene glaciations have important effects on the patterns of spatial distribution and genetic structure of organisms. Glacial refugium is the area where plants and animals could survive the severity climate and physical condition during glaciations, in particular the last glaciation. Studies on the glacial refugia and postglacial recolonization patterns can help reveal the relationship of different flora and fauna, understand speciation, and improve biodiversity conservation. In recent years, there are increasing interests on the glacial refugia because of the following reasons: First, glacial refugium is the source of postglacial redistribution, so studying glacial refugia and postglacial recolonization pattern can help to reveal the history and structure of different populations. Secondly, isolation between glacial refugia provides a very important mechanism leading to the development of new species or subspecies (speciation). Finally, glacial refugia are usually key regions for long-term maintenance of biodiversity and sites needing prior conservation because of their abundant biodiversity. Usually, glacial refugia are identified according to the endemism of flora and fauna based on the hypothesis that isolation between glacial refugia will lead to speciation especially with low recolonization ability. It is also obtained from some other indirect evidences, such as palynology, paleoclimatology, palaeobotany and fossils. Recently, molecular markers, in particular SSR, RFLP (or PCR-RFLP) and allozyme markers, have proved powerful in identifying the refugium and tracking the colonization route. More organelle genome (mtDNA and cpDNA) markers are used because the organelle genome is in maternal or paternal inheritance without recombination while nuclear markers are too complex and are affected by too many factors. As a consequence the initial geographical structure is probably retained longer for organelle genome than for nuclear genome markers. At present, studies on glacial refugium and postglacial recolonization using molecular genetic markers are mostly focus on the species of Northern America and Europe. Many studies revealed that the Bering Sea region, high-altitude zone of North Pole, the southern Appalachian Mountains and the southern Rocks Range are the main refugia for flora and fauna of North America. Poke milkweed (Asclepias exaltata), Dryas integrifolia, Microtus longicawdus, and Arctic grayling (Thymallus arcticus) are the species representing four types of glacial refugia and recolonization routes in Northern America. In Europe, the three southern peninsulas, i.e. Iberian, Italion and Balkan, are three important refugia for terrestrial flora and fauna during Pleistocene glaciation. Three broad patterns are evident for terrestrial species in Europe, and the grasshoper (Chorthippus parallelus), the hedgehog (Erinaceus europeus/concolor) and the brown bear (Ursus arctos) may serve as paradigms as Hewitt (2000) indicated. The refugia and recolonization routes of aquatic species, especially freshwater fishes are distinctly different from those of terrestrial creatures. The area of Danube middle and backward positions is usually thought of glacial refugia of freshwater fishes. When Pleistocene glaciations ended, the populations of the freshwater fishes in Danube recolonized to the north of Europe through Danube or other newly formed temporary rivers. During Pleistocene glaciations, China provided some key refugia for relict rare species because of its complex topography and many huge mountains of west-east direction. However, few studies have been done to identify the refugia. Due to the influences of human activity, many potential glacial refugium areas, that had high biodiversity before, now are facing destructive threat. It is urgent to pay attention to the glacial refugia. We propose the following researches should be carried out in priority: (1) studies on glacial refugia and recolonization routes of typical relict species, and (2) roles of key areas, such as the Crossing Ranges, the Southern Mountain Ranges, and the border area of Hunan, Hubei and Chongqing, during Pleistocene glaciations.