Descriptions of problem-solution ‘co-evolution’ either explicitly or implicitly draw an analogy between processes of design and processes of biological evolution. Analogies of this kind are common in research because of their potential to assist in explanation and discovery. However, reviewing the design literature reveals that the discussion of design co-evolution has become disconnected from the biological analogy on which it is founded, and from which other disciplines draw. Here, I explore the function of the co-evolution analogy, provide an illustrative example from biology, and explore the varieties of co-evolution to which design might be compared. By doing so, I propose two possible directions for expanding the design co-evolution concept: (i) examining what co-evolves in addition to, or instead of, problems and solutions, and (ii) examining the different levels at which co-evolution occurs. Both of these proposals are illustrated with a variant of the design co-evolution diagram.