What environmental pressure would cause primitive fish to adopt the genetic capacity to live on land?

Decrease in water/oxygen levels of the aquatic environment

Explanation:

The environmental pressure that would cause primitive fish to adopt the genetic capacity to live on land would be a decrease in water levels of the aquatic environment.

A constant decrease in water level that keeps exposing the fish to dry land will enable natural selection to start selecting for traits that will enable them to survive on land, especially, the development of lungs in the place of gills, the capacity to walk instead of swimming, and the capacity to avoid desiccation.

A decrease in water level will mean a decrease in the amount of oxygen available for gill's respiration. Respiration is an important process for any organism to stay alive.

A decrease in water level also means an exposure to sun. The heat from the sun could cause death by desiccation.

A decrease in water level means that movement (through swimming) will be limited. Movement is necessary for feeding and other biological activities.