Variation in a species gives its organisms of the same species a varied set of attributes such as size, hair colour, feather length, heart rate, fur thickness, etc. As an example lets look at the amount of fur on a pack of wolves living on an Island. The temperature of this island changes sometimes. If all the wolves have very thick fur they will be fine when the island is cold, but when the island heats up they will all overheat and die. If all the wolves have thin fur they will be fine when the island is very hot, but when the island gets cold they will all die. If however some wolves have thick hair and some wolves have thin hair no matter what happens some wolves will be able to survive. If the change in temperature happens slowly enough the variation will allow wolves to evolve to suite the new temperature because fur thickness is passed down to children, and wolves need to stay alive for a number of years before they are ready to have children.