Survival of the whitest, according to an article in BBC Earth News 6 November 2009.
The American black bear Ursus americanus has a uniformly black coat and lives mainly in forested areas of USA and Canada. However, on two islands off Western Canada 20 to 30 percent of black bears have white coats. These bears are known as Kermode bears or “spirit bears” and the white fur is the result of a recessive gene. White bears occasionally occur in the mainland populations, but are rare. This is assumed to be because white bears are not able to camouflage themselves in the shadowy forests as black coated bears can, so they get killed off by grizzly bears and wolves. Island bears are not exposed to these predators, but this alone would not explain the high number of white bears. Being white should confer some advantage in order to maintain the high rate of white genes.
Dan Klinka and Thomas Reimchen from Department of Biology, University of Victoria, British Columbia studied the feeding habits of island bears. During the autumn the bears hunt salmon that swim up the rivers on the islands. Salmon provide a rich source of protein and other nutrients for the bears, and a salmon-rich diet enables the bears to survive the winter and raise more young. Klinka and Reimchen found that black and white bears were equally efficient at catching salmon when it was dark, but the white bears caught more salmon during the day.
The researchers suggested that white bears were less visible to the salmon in the sunlight. They tested this theory by dressing themselves in white and black clothing and found that salmon were more likely to avoid them when they were dressed in black. Thomas Reimchen commented: “We hadn’t expected such a clear result as we got with our experimental work. The salmon were twice as likely to return to the area with the white costume than the dark costume.”
Editorial Comment: This is an example of natural selection, and the preservation of a favoured race in the struggle for life, to paraphrase Darwin’s book title, but not of evolution. Natural selection is an ecological process that works on already existing living things. The white gene is either a defective gene or a lost gene in the biochemical pathway needed to make the normal black pigment in the bears’ fur. It works in the white bears’ favour only on islands where there are no predators, and the main source of protein is salmon.
These white bears are variation in kind, brought about by genetic degeneration that gives them a survival advantage in a particular ecological niche. The biological processes that Darwin described in his book – selection, struggle and survival, are all real, but none of them explain the origin of any species.
Evidence News 17 February 2010
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