Answer
Population density could be variable enough to support the persistence of both alleles in one population.
Alternately, population density could be consistent for each community but variable between them, leading to some wild populations with $for^R$ and some with $for^s$.
Work Step by Step
For the presence of both alleles within the same population, the population density would have to be supportive of both behavior patterns. This indicates either a constant balance in density that supports both kinds of activity or a fluctuating density that alternately favors one or the other but changes often enough for both to survive.
