Some question the very existence of species, and believe that individuals or interbreeding populations are the only population system with any objective reality. This concept arose out of the philosophy of nominalism, arguing that only individuals are real and that classes of any kind (as species, genera, or families) are artificial constructs. For example, Burma (1954) stated: species are highly abstract fictions. Levin (2000) likewise argued that only the local population is the unit of evolution, and species are artificial. Some evidence supported nominalistic concepts. Ehrlich and Raven (1969) documented many cases of reduced gene flow in both plants and animals that would preclude any cohesive force to maintain species. They contended: Selection alone is both the primary cohesive and disruptive force in evolution for sexual organisms it is the local interbreeding population and not the species that is clearly the evolutionary unit of importance.
Rieseberg and Burke (2001) countered this view, arguing that prior studies grossly underestimated levels of gene flow, and that only very low rates of gene flow are actually needed for the diffusion of strongly advantageous alleles needed to maintain species integrity.