FIGURE 27.5. Skull of the polar bear (Ursus maritimus). From top to bottom:
lateral view of cranium
lateral view of mandible
dorsal view of cranium
ventral view of cranium
dorsal view of mandible. Click image to enlarge.
Despite the evidence of population segregation from marking, survey, and radiotelemetry data, initial evaluations using genetic techniques suggested small differences among polar bears in different geographic regions. Such small differences might be expected under Pedersen's (1945) hypothesis of a globally wandering panmictic polar bear population, but not in light of current knowledge of movements. Using protein electrophoresis, Larsen et al. (1983b) found little variation in allozymes among polar bears. They concluded there was no reason to reject a hypothesis of one common polar bear population worldwide. Larsen et al. (1983b) assumed that high gene flow across the Arctic and strong common selective pressures reduced variation among polar bears. Supporting that concept, Durner and Amstrup (1995) recorded the movement of a radio-collared polar bear from near Prudhoe Bay, Alaska, to Greenland. That bear was only 1 of 106 equipped with platform transmitter terminals or satellite radio-collars between 1985 and 1992. Relatively few such movements, however, would be required to genetically homogenize polar bear populations worldwide (Paetkau et al. 1995).
Corroborating the work of Larsen et al. (1983b), Cronin et al. (1991) found little mitochondrial DNA (mtDNA) differentiation among bears of the Beaufort and Chukchi seas. Mitochondrial DNA and protein electrophoresis, however, may have a relatively low ability to resolve genetic variation among populations (Scribner et al. 1997). Therefore, the absence of differences in these markers may not be surprising.
More recent studies using highly variable nuclear genetic markers called microsatellites have resolved differences among polar bears living in different areas. Paetkau et al. (1995) found significant differences in genetic distances among members of four Canadian populations of polar bears. They concluded that the long-distance movements of polar bears have not resulted in complete genetic mixing of populations. Furthermore, Paetkau et al. (1999) reported significant correlations among movement data and genetic data from polar bear populations worldwide. They found greater differences among populations in the Canadian Arctic than among populations surrounding the polar basin. Such contrasts fit well with observed differences in movement patterns in these areas (Amstrup et al. 1986, 2000; Messier et al. 1992; Amstrup and Gardner 1994; Bethke et al. 1996; Scribner et al. 1997).
Genetic management units that correspond with boundaries defined by radiotelemetry have now been identified for most polar bear populations (Paetkau et al. 1999). The correspondence between movement data and recent genetic data allows managers to make better decisions about harvest or other human activities that could have population-level impacts.
Comparisons of the relative genetic variability among putative populations of different bear species are difficult because literature on brown, black, and polar bears has not presented easily comparable or consistent measures of interpopulation genetic variation. Nonetheless, microsatellite data that can be compared suggest there may be less genetic variation among populations of polar bears than among populations of black bears and brown bears (Paetkau et al. 1995, 1999). Paetkau et al. (1999) also found genetic distances among polar bear populations were at the lower extreme of the distances reported for the gray wolf (Canus lupus), another widely distributed carnivore.
Evidence from patterns in mtDNA also may hint at somewhat less genetic variation among polar bear populations than among populations of other bears. Cronin et al. (1991) reported only one basic polar bear mtDNA lineage, whereas black and brown bears each have two very divergent lineages. The older species (black and brown bears) appear to have more genetic variation across their ranges than the more recently derived polar bears.
Greater morphological variation among populations of brown bears (e.g., very large individuals, such as those living on Kodiak Island and coastal Alaska, vs. smaller interior or arctic bears) also appears to reflect more genetic variation than is present among polar bears (Stirling and Derocher 1990; Talbot and Shields 1996a, 1996b). Morphological variation among polar bears is minimal throughout their range. Paetkau et al. (1999) concluded from the relatively small genetic distances and absence of major discontinuities among polar bear populations that all polar bears belong to one evolutionary lineage at this time. Over long periods of geologic time there has been a considerable amount of genetic exchange across the range of polar bears, resulting in low levels of population differentiation.
Although polar bears show less genetic variation among populations than do other bears, genetic variation within populations of polar bears appears to be similar to that within populations of black and brown bears. Paetkau and Strobeck (1998) concluded that polar bear populations were less variable than brown bears, which were less variable than black bears. When levels were averaged over several putative populations of each species, however, microsatellite heterozygosity within populations was 0.68 for polar bears (Paetkau et al. 1999), 0.66 for brown bears, and 0.72 for black bears (Paetkau et al. 1997), suggesting little difference. This pattern was also observed when two functional genes were compared. Considerable allelic variation in DNA sequences at thecasein and major histocompatability complex
(MHC) DQ? genes was observed in polar, brown, and black bears (M. A. Cronin and S. C. Amstrup, unpublished data) and no species appeared more variable than the others. It is thought that genes for ??-casein and the MHC are highly conserved because they influence milk quality and production and disease resistance. The functional importance of these genes may have led polar bears to retain their ancestral variability.
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