There shall be elected for each county, by the qualified voters, a County Clerk, who shall hold his office for four years, who shall be clerk of the County and Commissioners Courts and recorder of the county, whose duties, perquisites and fees of office shall be prescribed by the Legislature, and a vacancy in whose office shall be filled by the Commissioners Court, until the next general election; provided, that in counties having a population of less than 8,000 persons there may be an election of a single Clerk, who shall perform the duties of District and County Clerks.