The bass guitar took over from the double bass in the 1960s, and as the decade progressed the term "rhythm section" as used in a pop music context often came to refer to just the bass and drums.
The bass guitar took over from the double bass in the 1950s, and had almost completely taken over the bass role in the 1960s. As the 1960s progressed the term "rhythm section" as used in a pop music context often came to refer to just the bass and drums.