Similarly, natural selection simply means the differential survival of different forms of organism in a species. If the differences in form have no genetic basis then natural selection will not lead to a change in allele frequencies, and therefore evolution. For example, if there was natural selection against pink flamingos, which get their colour from their environment, not their genes, then this would not lead to the evolution of a new form in the population (unless there is a genetic character for tending to go and eat only food that enables them to turn pink).
Darwin’s genius was to realise that evolution by natural selection was a way for adaptations to appear. To get this to occur you need variability in a population, for that variability to have some genetic basis and for it to lead to differential survival. With that combination of three factors, and sufficient time, you end up with the amazing variety of life we have on the planet.