<Tr> <Td> Physiological example </Td> <Td> Bones / Umbilical cord </Td> <Td> Small bones of the inner ear </Td> <Td> White color of bones / Belly button </Td> <Td> Bumps on the skull, convex or concave belly button shape </Td> </Tr> <Tr> <Td> Psychological example </Td> <Td> Toddlers' ability to learn to talk with minimal instruction </Td> <Td> Voluntary attention </Td> <Td> Ability to learn to read and write </Td> <Td> Variations in verbal intelligence </Td> </Tr> <P> One of the tasks of evolutionary psychology is to identify which psychological traits are likely to be adaptations, byproducts or random variation . George C. Williams suggested that an "adaptation is a special and onerous concept that should only be used where it is really necessary ." As noted by Williams and others, adaptations can be identified by their improbable complexity, species universality, and adaptive functionality . </P> <P> A question that may be asked about an adaptation is whether it is generally obligate (relatively robust in the face of typical environmental variation) or facultative (sensitive to typical environmental variation). The sweet taste of sugar and the pain of hitting one's knee against concrete are the result of fairly obligate psychological adaptations; typical environmental variability during development does not much affect their operation . By contrast, facultative adaptations are somewhat like "if - then" statements . For example, adult attachment style seems particularly sensitive to early childhood experiences . As adults, the propensity to develop close, trusting bonds with others is dependent on whether early childhood caregivers could be trusted to provide reliable assistance and attention . The adaptation for skin to tan is conditional to exposure to sunlight; this is an example of another facultative adaptation . When a psychological adaptation is facultative, evolutionary psychologists concern themselves with how developmental and environmental inputs influence the expression of the adaptation . </P>

An explanation for individual differences in behavior from an evolutionary perspective is that