Just to add to point 1: "lower than" is generally used in comparing 2 subject's rank or position.
Ex: A colonel has lower rank than a general
Ex: Marc is in the lower echelon of the company
"lower" by itself can also mean "southern"
Ex: The lower St-Laurent
As for "of", I agree with Venksune that it is needed because it is the start of a prepositional phrase. It provides additional information on "population"
Ex: The population of bear
Ex: The population of more than two hundred thousands