Variation, adaptation, fitness, and natural selection address distinct explanatory questions within evolutionary biology. They can be related in particular evolutionary cases, but their presentation together does not establish a fixed sequence among them.
Variation concerns biological differences arising or present within continuing organisation. Developmental processes, genetic change, behaviour, ecological interaction, and other processes can contribute to such differences. Identifying variation does not by itself determine its consequences for viability or evolutionary continuity.
Adaptation concerns continuity-preserving reorganisation under changing conditions. Adaptive activity can affect evolutionary outcomes, but adaptation is not thereby established as a necessary intermediary between variation and fitness or natural selection.
Fitness concerns differential historical continuity. It characterises differences in continuity among biological variants or forms of organisation without, by itself, specifying the causal processes that produce those differences.
Natural selection concerns differential historical stabilisation. It addresses the historical differentiation associated with relevant biological differences across generations without itself explaining how every such difference arose or was maintained.
Inheritance can also be relevant where the cross-generational reconstitution of biological differences matters to the evolutionary explanandum. Its relevance does not make inheritance the first stage of a universal sequence leading through variation, adaptation, fitness, and natural selection.
These concepts therefore perform different explanatory roles. Their relations must be established for the biological case under investigation rather than inferred from their ordering within APS. Conceptual or expository ordering does not by itself establish temporal succession, causal priority, necessary dependency, or a universal pathway to evolutionary transformation.