‘The processes of evolution’ introduces the main causal processes of evolution. Evolutionary change ultimately relies on the mutations of organisms. These are caused by stable changes in the genetic material, transmitted from parent to offspring. Other processes must then cause mutations to increase in frequency within the population. These include natural selection and the random changes resulting from selectively neutral variability, which is called genetic drift. The combined effects of mutation, natural selection, and genetic drift cause changes in the composition of a population. Over a sufficiently long period of time, they alter the population's genetic make-up, and can change the species' characteristics from those of its ancestors.