Disciplinary variation in academic writing has been explored for the most part by comparing a particular genre, such as the research article, across different disciplines. However, genre theorists have not systematically studied relationships among related genres. It is argued in this article that a study of relationships among related genres from different disciplines can inform us about the differences in academic writing across disciplines. The generic structure of research article introductions and abstracts, which form a genre set, from two related fields, Conservation Biology and Wildlife Behavior, was compared. Findings indicate that research article introductions and abstracts in Conservation Biology bear a greater similarity in function and organization than the same two genres in Wildlife Behavior. This study reveals that disciplinary variation in academic writing is not just manifested in generic structure but also in the relationship among genres.