The answer should be become less common in a population.
Natural selective means it let a specific organisms with the characteristics that are more favorable to the environment have a greater chance to survive and pass their genes to the next generation.
For example, there is black moths and white moths. Black moths can camouflage themselves into the environment, meanwhile, white moths are more likely to be seen and caught by predators. Therefore, white moths has a less desirable characteristic and eventually their number would become less because more of them are caught by predators.
But these organisms with the less diserble characteristics won't completely disappear from the population. When their number become less, they eventually know how to hide themselves or protect themselves and will do their best to increase their population.
