what is the importance of the evolution of fresh water fish when fresh water represents only a small percentage of the earth's water resources

Fresh water represents only a small part of the water resources of the Earth. the fishes present in these ecosystem have high economical values and are beneficial to mankind. The evolution of these fishes is important because it will allow genetic varieties to arise in these fishes. Hence, these fishes will be able to survive and reproduce. Also, some of these fishes might adapt to live in salty environments in the course of evolution.