The civil service commission shall consist of five members or, if so resolved by the board of supervisors, seven members, each of whom shall hold office for a term of four years and until his successor is appointed and qualified. The members shall be selected from among the qualified electors of the county. Vacancies occurring in the commission shall be filled by appointment by the board of supervisors for the remainder of the unexpired term only. The civil service commission may be designated as a merit board.