Adaptationism is the view that many physical and psychological traits of organisms are evolved adaptations. Adaptationists perform research to try to distinguish adaptations (e.g., the umbilical cord) from byproducts (e.g., the belly button) or random variation (e.g., convex or concave shape of the belly button).  George Williams' Adaptation and Natural Selection (1966) was highly influential in its development, defining some of the heuristics, such as complex functional design, species universality, and reliability used to identify adaptations.