East Greenland and Barents Sea polar bears (Ursus maritimus): adaptive variation between two populations using skull morphometrics as an indicator of environmental and genetic differences

摘要

The polar bears of East Greenland and those of the Barents Sea area are currently being managed as two separate management units (IUCN PBSG 2010). Paetkau et al. (1999) studied the genetic structure in 16 of the world’s 19 predefined polar bear populations, based on neutral genetic markers; very low level of genetic differentiation was found between the East Greenland and Barents Sea populations, although the allele frequency distributions qualified the two populations as separate management units (MUs) (Moritz 1994). However, recent observations have questioned the degree of separation between the two populations, suggesting a certain overlap (Born et al. 2009). Still, differences in space use (Born et al. 2009) and skull growth rates (Bechsh?ft et al. 2008a), as well as subtle differences in skull, teeth, and bone characters have been found between polar bears from East Greenland and Barents Sea (Sonne et al. 2007a, 2007b; Bechsh?ft et al. 2008b, 2009).One of the main goals of conservation genetics is the identification of evolutionary significant units (ESUs) – and MUs and the preservation of genetic diversity, which should allow the evolutionary processes of natural selection and adaptation to continue in the future. The identification of separate ESUs was originally recommended based on both ecological and genetic data, although focus has later shifted to being mainly on the assessment of neutral genetic variation (Waples 1991; deGuia and Saitoh 2007). However, a broader definition of ESUs including non-neutral markers and adaptive variation would be more appropriate (deGuia and Saitoh 2007). Neutral molecular marker loci provide only little insight into adaptive variation, unless a large fraction of the non-neutral markers are tightly linked to the relevant quantitative trait loci. Alternatively, neutral markers can provide useful information in small populations where most of the fitness variation is neutral. However, a lack of molecular divergence among populations at neutral loci is potentially uninformative, as it cannot exclude local adaptations (Lynch 1996). Neutral genetic markers such as microsatellites have a well-known inheritance but may not be useful in studies of local adaptation, since there is often a limited correlation between neutral and selective variation in natural populations (Meril? and Crnokrak 2001). Genetic differences, on the other hand, could potentially be measured by proxy instead, as e.g. morphometric variation in physical traits.Knowledge of local adaptation and adaptive potential of various wildlife populations is becoming increasingly relevant due to impacts on the environment from i.e. climate change and pollution, which could potentially favour the more adaptive species. Divergent natural selection due to spatially varying environments is expected to promote adaptive evolutionary responses (Kawecki and Ebert 2004). Hence, the evolutionary outcome is dictated by the relative strength of natural selection (Endler 1986).Selective forces influence populations in various parts of their range differently (Andersson 1994). Demonstration that a population is adapted to a given environment is also important (Pertoldi et al. 2007, 2009). Acquiring direct evidence involves either comparison of fitness among populations in local and foreign environments (Kawecki and Ebert 2004).Presently more information is thus wanted on the degree of exchange between the polar bear populations of East Greenland and the Barents Sea. The main aim of the present study was therefore to compare differences in size and shape of East Greenland and Barents Sea polar bear skulls, exploring the degree of morphometric differentiation between the two populations.