Mayr wrote in "What evolution is": "most treatments of evolution are written in a reductionist manner in which all evolutionary phenomena are reduced to the level of the gene. An attempt is then made to explain the higher-level evolutionary process by "upward" reasoning. This approach invariably fails. Evolution deals with phenotypes of individuals, with populations, with species; it is not "a change in gene frequencies."" As he says, I understand most reductionist scientists (not philosophers) consider reductionism as the above.