 Suppose, nevertheless, that a trait occurs in all the members of a species. That trait is the essence of a species only if it is unique to that species. Yet organisms in different species often have common characteristics. Again, biological forces work against the uniqueness of a trait within a single species. Organisms in related species inherit similar genes and developmental programs from their common ancestors. These common stores of developmental resources cause a number of similarities in the organisms of different species. Another source of similar traits in different species is parallel evolution. Species frequently live in similar habitats with comparable selection pressures. Those selection pressures cause the prominence of similar traits in more than one species. The parallel evolution of opposable thumbs in primates and pandas is an example.