 But on a typical way of instantiating this rather abstract model of natural selection, one can identify what plays the role of the individuals in this model (organisms) and what plays the role of the populations (species). This popular view treats organisms as the units of selection and species as the units of evolution. Species evolve, owing to the differential survival and reproduction of organisms. This emphasis on organism-level selection and species-level evolution is another which dates back to Darwin’s Origin of Species, although Darwin also countenanced family selection (Darwin 1859), sexual selection (Darwin 1871) and group selection (Stauffer 1975). Some early proponents of the modern synthesis (e.g., Mayr 1942, 2001) also shared this basic picture in which selection acts on organisms, while species evolve. To propose that selection occurs at the level of species as opposed to that of organisms, is to suppose that not only can organisms act as the units of selection but that species can too.