Four conditions are needed for natural selection to occur: reproduction, heredity, variation in fitness or organisms, variation in individual characters among members of the population. If they are met, natural selection automatically results.
Ur welcome
Explanation:
The definition of watershed is
"any point on uphill from where water flows downstream wither in the form of river, stream or rain water".
From the definition we can say that watershed (either natural or man made) would me anything that is present in that area where the water flows downstream to houses, parks, and dams.
I have attached picture for better understating.