 The disagreements over the prevalence of drift and selection began almost immediately after Wright and Fisher incorporated drift into their evolutionary models (see Provine 1986 for an extended discussion of their disagreements, to which this discussion is very much indebted). Wright (1931a, 1932), drawing on his experience with animal breeding, developed the Shifting Balance Theory (SBT), which consisted of three phases; these have been understood either as empirical claims about actual conditions (Provine 1986) or descriptions of the ideal conditions for evolution (Skipper 2002). Skipper provides a clear description of the SBT as Wright would eventually formulate it (see Hodge 2011 for discussion of early formulations):