Variation, differential retention, and reproduction of what was retained. Wherever those three appear together, you get design without a designer — and they appear together at half a dozen levels inside a single human being.
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Selection is not only a fact about biology.
Biology gives us the most famous case: organisms vary, some variants leave more descendants than others, and retained structure is copied forward. Over enough time, design appears without a designer.
But the same shape appears wherever three ingredients show up together.
There must be variation. There must be some difference in what gets retained. And there must be some way for retained structure to influence what comes next.
When those three are present, a system can improve without knowing the answer in advance. It can try many possibilities, keep some, discard others, and let the retained structure bias the next round.
That is why selection keeps reappearing inside a single life.
Genes are selected. Immune cells are selected. Neural connections are selected. Behaviours are selected. Ideas are selected. Scientific conjectures are selected. Machine outputs can be selected. Different mechanisms, different substrates, different timescales — but the same abstract operation.
Variation. Retention. Inheritance of what was retained.
Selection all the way down.
Here is the movement's stack in prose.
At the genetic level, variants are alleles and organisms. What retains them is differential reproduction. This is natural selection in the ordinary sense.
At the immune level, variants are lymphocyte clones. What retains them is antigen binding and expansion. The immune system does not know in advance which invader will arrive. It generates variety and amplifies the clones that fit.
At the neural level, sense A, structural, variants are synapses, circuits, and neuronal groups. What retains them is activity-dependent survival, strengthening, weakening, and pruning. The brain grows more than it keeps. Structure is produced, tested by use, and sculpted.
At the behavioural level, variants are emitted actions. What retains them is consequence: reinforcement, punishment, success, failure, relief, pain, reward, and social response. The animal tries; the world answers; future behaviour changes.
At the representational level, sense B, variants are ideas, models, images, hypotheses, stories, plans, and expectations inside a mind. What retains them is fit, usefulness, emotional grip, transmissibility, and sometimes truth.