Purpose is a human-dependent measure.
Natural selection explains the design-like nature of organisms and their characteristics, without any need to talk about final causes. On the other hand, no natural selection lies behind mountains and rivers and whole planets. They are not design-like. That is why teleological talk is inappropriate, and why the Gaia hypothesis is so criticised. And overall that is why biology is just as good a science as physics and chemistry.
This is wrong-minded. There is no fundamental difference between the evolving form of a river, the minerals in a rock, or the phenotype of a species. They are all equally 'design-like'. It is simply a matter of shifting perspective and what you define to be the 'entity', and as a result, what is left for the 'environment'.