Wednesday, January 30, 2013

Spirit Bears

A white mother Spirit bear and a black cub offspring; the father must have been black and the cub is a heterozygote for the coat color polymorphism. This picture really reminds me of the old Polar Bear joke which is not really suitable for printing here...

It's Carl Zimmer again with a really nice practical example of one of the simulations we will look at in class tomorrow - the interplay between genetic drift and natural selection: Snow Coyotes and Spirit Bears

The Bear part of the story is based on this paper in the journal 'Evolution': POPULATION GENETICS OF THE WHITE-PHASED “SPIRIT” BLACK BEAR OF BRITISH COLUMBIA

The Spirit (or Kermode) bear is a white-phased black bear found on the northwest coast of British Columbia, and is one of the most striking color polymorphisms found in mammals. A single nucleotide polymorphism at the melanocortin 1 receptor gene (mc1r) locus is the cause of this recessive w variant. Recently, evidence suggests that the white color provides a selective advantage during salmon hunting. Here we examine the effects of favorable selection, gene flow, genetic drift, and positive-assortative mating in an effort to understand the establishment and maintenance of this polymorphism and the observed heterozygote deficiency for mc1r but not for microsatellite loci. It appears that genetic drift was important in the establishment of the w allele and that the selective advantage was important to counteract immigration from populations without the w allele. Positive-assortative mating can result in a deficiency of heterozygotes but needs to be quite high to result in the large deficiency of heterozygotes observed, suggesting that other factors must also be contributing. Examination of population genetic factors, singly and jointly, provides insight into the establishment and maintenance of this unusual polymorphism.

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