Viability and fitness are related, but APS distinguishes them because they address different explanatory questions.
Viability concerns the maintenance of viable organisation under present conditions. A living system remains viable while it sustains the organisation required for continued existence despite internal change and environmental challenge.
Fitness concerns differential historical continuity. It characterises differences in continuity among biological variants or forms of organisation across generations without, by itself, specifying the causal processes responsible for those differences.
Relations between viability and fitness can be biologically important. Features that affect present viability may also affect differential historical continuity in particular cases. But this relation must be established for the explanandum concerned rather than inferred from the concepts’ positions within APS.
Viability therefore should not be treated simply as an explanatorily prior stage leading to fitness, nor should fitness be treated as viability extended through time. They distinguish present viability from differential historical continuity while allowing their biological relations to be investigated independently.