Complex sentence: With the advent of rock-and-roll music, WEIM evolved into a Top 40 station by the late 1950s, a format that would remain throughout the 1960s and 1970s.
Split into simpler sentences: With the advent of rock-and-roll music, WEIM evolved into a Top 40 station by the late 1950s. It continued as a Top 40 station throughout the 1960s and 1970s.