Answer:
For natural selection to work there must be variation in the population for it to act upon and so ‘choose’ the most fit genes for a particular situation. Populations tend to produce more offspring than the environment can support, and the inevitable consequence of this overproduction of offspring is a struggle for survival. Natural selection creates these selection pressures so the organisims best suited for the environment are more likley to survive and therefore pass on their genes to their offspring.
Natural selection can only happen in a population rather than an individual becasue it is the whole population's characteristics culmalitvley changing to better suit the environment. By passing down favourable traits to their offspring. It cannot happen in an individual because they cannot change their traits to better suit the environment, only pass on their traits to their offspring. A change due to natural selection can take generations