Distribution and abundance reflect evolutionary and geologic history
Events in the evolutionary and geologic history of Earth have had a profound effect on where organisms live today. Why, for example, are polar bears (Ursus maritimus) found in the Arctic but not in Antarctica? Polar bears hunt on ice packs and eat seals, both of which abound in Antarctica.
Part of the answer to our question can be found in the evolutionary history of these bears. Fossils and genetic evidence indicate that polar bears evolved from brown bears (Ursus arctos) in the Arctic (Lindqvist et al. 2010); hence U. maritimus is found in the Arctic because the species originated there. As for their absence from Antarctica, although polar bears can travel over 1,000 km in a year, it appears they cannot or will not cross the tropical regions that separate the Arctic from Antarctica. Thus, the distribution of polar bear populations is influenced by evolutionary history and dispersal as well as by the presence of suitable habitat.Geologic history plays a key role in some curious distribution patterns that puzzled biologists for nearly 100 years. Consider Alfred Russel Wallace's observation that the animals of a region can differ considerably across relatively short geographic distances (Wallace 1860). The mammal communities of the
Philippines, for example, are more similar to those in Africa (88% overlap at the family level) than they are to those in New Guinea (64% overlap), despite the fact that Africa is 5,500 km away and New Guinea is only 750 km away. No explanation for this and other similar observations could be found until the discovery of continental drift, the gradual movement of continents over time. This discovery led to the realization that the Philippines and New Guinea are on different tectonic plates and have been in close geographic contact for a relatively short period of time.