William James, John Dewey, and Charles Sanders Peirce were instrumental in founding functional psychology, which draws heavily from Darwin's theory of evolution by natural selection. This theory suggests that individual traits, including behaviors, are adapted to their environments through natural selection. At the heart of functionalism is the concept of adaptation, meaning that a trait enhances an individual's chances of survival and reproduction.

James envisioned psychology's role as investigating the function of behavior in the world, marking the essence of functionalism. This perspective focuses on how mental activities enable an individual to adapt to their environment, contrasting with structuralism's emphasis on the components of the mind.

Functionalists aimed to uncover the adaptive purposes of psychological traits like thoughts, feelings, and behaviors, exploring "why" questions rather than the "what" questions of structuralists. Rejecting structuralist methods, James promoted introspection as a means to understand the continually changing "stream of consciousness." His landmark book, The Principles of Psychology, broadened the public's understanding of psychology, introducing functionalist concepts.

Influenced by Darwin, functionalists applied the theory of natural selection to psychological traits, positing that these, like physical traits, evolved for survival benefits. They endeavored to identify the adaptive functions these traits served, likening their role to that of investigators unraveling the evolutionary purposes behind psychological characteristics.

Functionalism, established by figures like James, Dewey, and Peirce, is rooted in the evolutionary concept that behaviors and psychological traits have adapted over time for survival. Functionalism contrasts with structuralism by focusing on the mind's overall operation rather than its parts, emphasizing the importance of understanding mental activities' adaptive roles.