There are elephants that have short and long trunks
Change in gene pool
The elephants with short trunks are not fit for the environment and are not able to get food and water , and aren't able to reproduce.
since the elephants with short trunks aren't fit for the environment the elephants with long trunks are the fittest for the environment they are able to survive and reproduce, continuing the trait of long trunks
The trait of long trunks is being carried out by the elephants. This trait is making the elephants fit for the environment