How does the creation of variations in a species promote its survival?

When the environmental conditions for a species change so much that the survival becomes difficult, the individuals of the species with variations only survive. For example- When drought conditions prevail on an island- harder seeds are produced in dry years and softer seeds are produced in wet years. Those birds that have variants with their beaks for consumption of type of seed available, only they survive and the rest are removed because they don’t get food. This variants help in the survival of species. If these variants would not have been available the entire species would be destroyed.