1. There must be a pattern involved.
2. The pattern must be copied somehow (indeed, that which is copied may serve to define the pattern). [Together, 1 and 2 are the minimum replicable unit -- so, in a sense, we could reduce six essentials to five. But I'm splitting rather than lumping here because so many "sparse Darwinian" processes exhibit a pattern without replication.]
3. Variant patterns must sometimes be produced by chance -- though it need not be purely random, as another process could well bias the directionality of the small sidesteps that result. Superpositions and recombinations will also suffice.
4. The pattern and its variant must compete with one another for occupation of a limited work space. For example, bluegrass and crab grass compete for back yards. Limited means the workspace forces choices, unlike a wide-open niche with enough resources for all to survive. Observe that we're now talking about populations of a pattern, not one at a time.
5. The competition is biased by a multifaceted environment: for example, how often the grass is watered, cut, fertilized, and frozen, giving one pattern more of the lawn than another. That's Darwin's natural selection.
6. New variants always preferentially occur around the more successful of the current patterns. In biology, there is a skewed survival to reproductive maturity (environmental selection is mostly juvenile mortality) or a skewed distribution of those adults who successfully mate (sexual selection). This is what Darwin later called an inheritance principle. Variations are not just random jumps from some standard starting position; rather, they are usually little sidesteps from a pretty-good solution (most variants are worse than a parent, but a few may be even better, and become the preferred source of further variants).
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