Except as may otherwise be provided under this act: (a) Every employer shall pay to each of his or her employes wages for all hours worked at a rate of not less than: (1) Two dollars sixty-five cents ($2.65) an hour upon the effective date of this amendment. (2) Two dollars ninety cents ($2.90) an hour during the year beginning January 1, 1979. (3) Three dollars ten cents ($3.10) an hour during the year beginning January 1, 1980. (4) Three dollars thirty-five cents ($3.35) an hour after December