In his later writings, Talcott Parsons tied his functionalist theory (notably the four so-called ‘systems problems’) to an evolutionary perspective, exemplified in his twin volumes on Societies (1966) and The System of Modern Societies (1971). He argued that, like biological organisms, societies progress through their ‘capacity for generalized adaptation’ to their environment. This is achieved mainly through processes of structural differentiation; that is, the development of specialized institutions to perform the social functions necessary to meet increasingly specialized needs. However, this increasing complexity then requires new modes of integration, in order to co-ordinate the new and more specialized elements. This is achieved through the ‘cybernetic hierarchy’, which involves increased information exchange or the growth of knowledge. (In this way culture comes to be the dominant influence on the social system in Parsons's work.)
Evolution is a move from traditional to modern societies, and progress can be charted by the development (structural differentiation) of evolutionary universals such as bureaucratic organization, money and market complexes, stratification, and the emergence of generalized universalistic norms. Each of these enables a society to adapt more efficiently to its environment (see ‘Evolutionary Universals in Society’, American Sociological Review, 1964).