If a majority of the votes cast on the question of adoption are in favor of adopting the optional form of government provided in sections 78.700 to 78.720, the existing council shall by ordinance divide the city into five wards of approximately equal population. The council shall then assign an arbitrary designation to each ward and shall cause a lot to be drawn for each ward designated from among the five lots, numbered one through five. The wards shall thereafter be known as wards one, two, three, four and five according to the lot drawn for each.