 There is a similar caution in discussions of species or clades as evolutionary or physiological individuals as there is with trait groups. One of the chief threads to the debate over species and clade selection also parallels that over trait group selection: are species or clades themselves really the agents of selection, the units that are being selected, or do they simply tag along for the ride, with selection operating exclusively on organisms and genes? Elisabeth Vrba (1984, 1989; Vrba &amp; Gould 1986), for example, has distinguished between species sorting and species selection, arguing that while a sorting of species may be the product of evolution by natural selection (see Barker &amp; Wilson 2010), this outcome is typically brought about not by species selection but by individual selection. On this view, species or clades may be a product of natural selection, and so in some sense evolutionary individuals, but they are not themselves agents in the process of natural selection. Rather, they are epiphenomena of that process, lacking the kind of agency that full-blown evolutionary individuals have.