  Weber (2005) examines the evolution of the gene concept by tracing changes in the reference of the term gene through the history of genetics. The reference or extension of a term is the set of objects to which it reference. Weber adopts a mixed theory of refence. According to mixed theories, the reference of a term is determined how the relevant linguistic community causally interacts with potential referents as well as how they describe potential referents. This theory leads Weber to pay close attention, not just to how geneticists theorized about genes or used the concept to explain phenomena, but also how they conducted their laboratory investigations. Following Kitcher (1978, 1982), he examines ways in which modes of reference changed over time.