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Their underlying power rivalry and their self-images as natural great powers and centres of civilisation and culture will continue to drive them to support different countries and causes . Genetic data ( from studies of protein electrophoresis and DNA ) were the primary evidence considered for the reproductive isolation criterion , supplemented by inferences about barriers to migration created by natural geographic features and human-induced changes resulting from artificial propagation and harvest . In unpopulated areas and over the oceans , a small amount of natural low altitude ozone is formed from sunlight driving reactions of natural volatile organic compounds ( VOCs ) from vegetation and natural nitrogen oxides ( NOx ) , primarily from lightning and forest fires . I decided to place them on natural forested pasture to do one of the first comprehensive and documented studies of domesticated horses returning to a wild environment .