All living things have characteristics that help them to survive. Individuals in a species show many differences. This variation is a result of problems inherited. There is competition between individuals for food, space and other resources. This leads to a struggle for survival. The individuals that can compete for all the vital resources are most likely to survive. This is known as 'Survival of the Fittest' or more properly 'Natural Selection. If variation gives an individul a grater ability to compete for resources, then they will have a better chance of survival.
E.G. A tortoise has a different neck to everyone else sohe can compete for food that is higher up.