 Because of this diachronic and synchronic variation in gene concepts, Weber speaks of “floating reference”, highlighting permanent change in the reference potential and shifts in reference. This fact, according to Weber, is explained by metaphysical considerations regarding the nature of biological objects. Evolution produces variation; this is true also for genetic material. As a result, genetic material exhibits kinds in the sense of classes of objects that share some causal disposition, but it can nonetheless be divided in various ways. Hence different historical and contemporary gene concepts identify different kinds depending on the methods and interests of researchers. Apart from the different ways to identify causally relevant classes of objects, any such class contains objects none of which is exactly like the other. The gene concept is thus comparable to the species concept (Weber 2014; see entry on  species).