Gaps and similarities are two important concepts used in Information System (IS) projects that deal with the evolution issue. The idea in using these concepts is to analyse what changes or what remains similar between two situations, typically the changed situation and the new one, rather than just describing the new situation. Although in the industry, the daily practice consists in expressing evolution requirements with gaps and similarities, little attention has been paid in research to better systematically define these two kinds of concepts so as to better support the expression of evolution requirements. This paper proposes an approach that combines meta-modelling with generic typologies of gap operators and similarity predicates. Our purpose is not to define yet another requirement modelling language. On the contrary, the two generic typologies can be adapted to existing modelling language such as Use Cases, I* and KAOS goal models, Goal/Strategy maps, Entity-Relationship diagrams, and Workflow models.