For example, a large gut might assist digestion. However, if the gut size increases sharply and in a disproportionate manner, the animal might not be able to outrun predators. Such an animal would quickly go extinct. This illustrates that one cannot look at single organs or subsystems in isolation. Rather, one must look at the whole organism, or at least attempt to do so.
Diets can temporarily change with habitat in the same species.
Staple foods vs. critical but more rare foods. The above reflect why examination of all the evidence of morphology, the fossil record, and paleoclimate are important in assessing adaptation. The form of digestive features can be affected by non- diet factors. If the character of interest varies only at a very high taxonomic level it may be extremely difficult to separate the favored explanation for the character from many other explanations associated with phylogenetic differences. Difficulty of untangling cause and effect.