During periods of a wet or Green Sahara, the Sahara and Arabia become a savanna grassland and African flora and fauna become common. Following inter-pluvial arid periods, the Sahara area then reverts to desert conditions, usually as a result of the retreat of the West African Monsoon southwards. Evaporation exceeds precipitation, the level of water in lakes like Lake Chad falls, and rivers become dry wadis. Flora and fauna previously widespread as a result retreat northwards to the Atlas Mountains, southwards into West Africa, or eastwards into the Nile Valley and thence either southeast to the Ethiopian Highlands and Kenya or northeast across the Sinai into Asia. This separates populations of some of the species in areas with different climates, forcing them to adapt, possibly giving rise to allopatric speciation.