Individual organisms within a species vary morphologically (size, shape, color), physiologically, behaviorally, and demographically. Some humans are tall, some are short; some are blond, some are brunette. Some trees in a species grow tall with narrow, compact crowns; others are more stocky with dense, wide-radius crowns. Some dogs in a litter are more active and aggressive than some of the others. These variations are of genetic interest in terms of whether they are heritable. They are of ecological and evolutionary interest in terms of the environmental influences that shape them, both nongenetically in the present and genetically over an evolutionary time scale.