It would be difficult for the natural selection to happen as the population decreases because greater the variety of traits in a population greater is their chance of survival by natural selection.
Step-by-step explanation:
The natural selection process requires four factors, these are overproduction, heredity, variation in adaptations and variations in their characters.
There are chances in small or isolated populations some random changes might happen which can affect the whole population so natural selection will not happen in this condition whole population will get polarised according to the change positive or negative whatever.