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NatureAs winter approaches, polar bears wait to hunt on sea ice in Kaktovik, Alaska; in late spring, grizzlies await caribou on the northern slope of Alaska's Brooks Range.

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Now PlayingNatureAs winter approaches, polar bears wait to hunt on sea ice in Kaktovik, Alaska; in late spring, grizzlies await caribou on the northern slope of Alaska's Brooks Range.0NatureExamples of how the rain forest acts as an engine of evolution.