Major shifts, that is shifts in the paradigms, are not very frequent - they would make scientific research unstable, as any result of a smaller study could be made invalid by a paradigm shift,
minor changes however are quite common - basically every scientific study adds to the common knowledge and this also can be considered a minor change.
Therefore the best answer is
Major
shifts in scientific views very rarely occur, but minor modifications
of prior knowledge are continually made based on new research.
