def is_population_monomorphous(population_array, population_size):
    """
    Determines if a population is monomorphous. 
    Returns
    -------
    int: the index of the strategy that in which the population is monomorphous, or None if it is not monomorphous.
    """
    max_index = population_array.argmax()
    if population_array[max_index] == population_size:
        return max_index
    return None