The long-term sustainability of research software is a critical challenge, as it usually suffers from poor maintainability, lack of adaptability, and eventual obsolescence. This paper proposes a novel approach to addressing this issue by leveraging the concept of fitness functions from evolutionary architecture. Fitness functions are automated, continuously evaluated metrics designed to ensure that software systems meet desired non-functional, architectural qualities over time. We define a set of fitness functions tailored to the unique requirements of research software, focusing on findability, accessibility, interoperability and reusability (FAIR). These fitness functions act as proactive safeguards, promoting practices such as modular design, comprehensive documentation, version control, and compatibility with evolving technological ecosystems. By integrating these metrics into the development life cycle, we aim to foster a culture of sustainability within the research community. Case studies and experimental results demonstrate the potential of this approach to enhance the long-term FAIR of research software, bridging the gap between ephemeral project-based development and enduring scientific impact.