 This entry initially distinguished two usages of “natural selection”, the focused and the capacious one, the focused one picking out a single formal element of the process captured by the capacious sense. There are some intermediate cases. Brandon offers this definition of selection: selection is differential reproduction that is due to differential adaptedness to a common selective environment (Brandon 2005: 160). This definition does not attempt to capture the entirety of Darwin’s process: while it identifies natural selection with a type of reproduction, there is no mention of inheritance or replication. Repeat a process specified to fit Brandon’s definition as often as you like and we still might not have something that makes adaptation or speciation explicable. But neither is Brandon’s definition of selection appropriate as an interpretation of a theoretical term in any formalism. Instead, that definition connects with an effort to develop a Principle of Natural Selection which would serve a seminal role in a qualitative version of Darwin’s theory. The Principle is formulated using probability talk along with the notion of fitness. The historical development of the Principle of Natural Selection, beginning with Brandon together with Beatty and Mills (Brandon 1978; Mills &amp; Beatty 1979), has been guided by debates over the interpretation of probability and fitness addressed elsewhere (Brandon 1990: ch. 1; Sober 2000; Campbell &amp; Robert 2005; entry on  fitness).