  There is, however, a way of formulating the theory in its modern guise which maintains an essentially Darwinian character.  Since there are a number of confirmed ways in which natural populations can evolve in the absence of natural selection, and since balancing selection, i.e. countervailing selection forces, may prevent a population from evolving in its presence, it is clear that establishing, by measuring different reproductive rates among its members, that the genetic make-up of a population has changed does not establish that natural selection was the source of that change; nor does the fact that no change has been measured establish that natural selection is not operative.  Population genetics and its associated models should be treated as the ‘kinematics’, not the ‘dynamics’ of evolutionary processes. That is, it is a way of establishing that a population either is or is not in equilibrium, and it provides sophisticated tools for measuring rates of change in a population across generations. Moreover, like the kinematics of any physical theory, if it establishes cross-generational change, it also tells us that there are causes to be found—the detailed contours of those measures may even provide suggestions as to where to look for those causes. What it cannot do on its own is provide knowledge of the forces at work.  To use language introduced by Elliott Sober, fitness, unlike natural selection, is causally inert. (For further information, see the entry on   population genetics.) 