How does the creation of variations in a species promote survival?
Gene variation arises in a population due to the mutation in genes. In a sexually reproducing organism, when this mutation happens in the germinal cell, mutate genes transfer into the next generation during reproduction. Erratic mutation eliminates immediately while useful and dormant variations stay in the population. When some external environmental factor comes into action these variation helps them in their survival.
For example, there are two varieties of moth (a butterfly), White and Black. The black moth came into existence through variation. Bird pick them and eat as their food. During the industrial revolution in the 20th century, there was an increase in the use of coal by the industries. It resulted in the formation of black soot which deposited on the trees. Now, white moths were easily distinguishable by contrasting black smoke and soot; birds almost ate all the white moth population. Here black moth survived. Later on, laws to curb pollution came into existence. It reduced the formation of soot and smoke. Now both types of moth exist in balance.