Asituation where all of the forces acting upon a population are in balance- allowing the populations to remain relatively constant is called

This condition is known as genetic equilibrium

In genetic equilibrium, there are a number of assumptions that are made in order to obtain the conditions that are described. Note that these assumptions cannot exist in real life and genetic equilibrium is a hypothetical state.

-No gene mutation
-Large population
-No immigration/emigration 
-No natural selection
-Random mating

Through these conditions, the occurrence of all phenotypes within an organism's population remains relatively the same.