The commission on legislative apportionment established by the 1963 state constitution shall consist of 8 electors, 4 of whom shall be selected by the state central committee of each of the 2 political parties whose candidates for governor received the highest vote at the last general election at which a governor was elected preceding each apportionment. If a candidate for governor of a third political party has received at such election more than 25% of such gubernatorial vote, the commission shall consist of 12 members, 4 of whom shall be selected by the state central committee of the third political party. One resident of each of the following 4 regions shall be selected by each state central committee: (a) the Upper Peninsula; (b) the northern part of the Lower Peninsula, north of a line drawn along the northern boundaries of the counties of Bay, Midland, Isabella, Mecosta, Newaygo and Oceana; (c) southwestern Michigan, those counties south of region (b) and west of a line drawn along the western boundaries of the counties of Bay, Saginaw, Shiawassee, Ingham, Jackson and Hillsdale; and (d) southeastern Michigan, the remaining counties of the state.