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

(N/A) Sometimes,environmental conditions change so drastically that the survival of a species becomes difficult. For example,if the temperature of water increases suddenly,most of the bacteria living in that water would die. Only a few variants resistant to heat would be able to survive. If these variants were not present,the entire species of bacteria would have been destroyed. Thus,these variations help in the survival of the species.
However,it is important to note that not all variations are useful. Therefore,variations are not necessarily beneficial for the individual organisms in all situations.