 This is what makes similarity of function an especially problematic criterion of homology (Abouheif et al. 1997). Because functional similarity is the appropriate relation for analogy, it is not necessary for analogues to have the same function as a consequence of common ancestry—similarity despite different origins suffices (Ghiselin 2005). Classic cases of analogy involve taxa that do not share a recent common ancestor that exhibits the structure, such as the external body morphology of dolphins and tuna (Pabst 2000). Thus, functional homology seems to be a category error because what a structure does should not enter into an evaluation of homologue correspondence and similarity of function is often the result of adaptation via natural selection to common environmental demands, not common ancestry.