 The second consequence implies that intrinsic similarity is not necessary for membership of the same species. There exists a high degree of variation in intraspecific morphology and genetic makeup. While members of kinds need not be intrinsically identical (e.g., there may be isotopic variation between samples of a chemical element), there will nonetheless be certain distinctive intrinsic natural properties common to all members of the kind (e.g., nuclear charge in the case of atoms of the same element). That, it is claimed, is absent for species. Thus there is no genetic material or sequence of genes that all and only members of the species Drosophila melanogaster possess, and likewise for all other species. Nor can we turn to larger-scale phenotypic properties (which may nonetheless be hidden), since evolutionary change may eliminate such features without a new species arising (Sober 1980). Furthermore, gradual change, even through speciation, means that species will not be categorically distinct (Criterion 6), which for Ellis (2001) is an additional reason to conclude that species are not natural kinds.