DNA rules specify that G always pairs with C, and A with T. But, when it’s all added up, the amount of G+C vs A+T content among species is not a simple fixed percentage or, standard one-to-one ratio. Biologists have now experimentally demonstrated that G+C composition is generally strongly favored by natural selection, regardless of the class of DNA, size of a species’ genome, or where the species is found on the evolutionary tree of life.