Ecological-evolutionary theory (EET) is a sociological theory of sociocultural evolution that attempts to explain the origin and changes of society and culture. Key elements focus on the importance of natural environment and technological change. EET has been described as a theory of social stratification, as it analyzes how stratification has changed through time across different societies. It also has been viewed as a synthesis of the structural functionalism and conflict theory. Proposed by Gerhard Lenski, the theory perhaps is best articulated in his book, Ecological-Evolutionary Theory: Principles and Applications (2005).His major collaborators, Jean Lenski and Patrick Nolan, also are said to have contributed to EET.