Fitness and natural selection are closely related, but they do not explain the same aspect of evolutionary change.
Fitness concerns differential historical continuity. It characterises differences in the continuity of biological variants or forms of organisation across generations without, by itself, specifying the causal processes responsible for those differences.
Natural selection concerns differential historical stabilisation. It addresses the historical differentiation associated with relevant biological differences without itself explaining how those differences arose or were maintained.
The distinction is important because differences in fitness and natural selection can be related without forming a fixed explanatory sequence. Identifying differential historical continuity does not by itself establish the causes of that difference, and characterising natural selection does not require fitness to function as a separate causal intermediary.
APS therefore treats fitness and natural selection as complementary but distinct explanatory concepts. Their biological relations must be established for the evolutionary case under investigation rather than inferred from their ordering within the APS architecture.