In this third and final post about the principles of natural selection introduced in Chapter 1 of Enrico Coen's Cells to Civilizations, The Principles of Change That Shape Life I include treatment of the fourth principle, reinforcement. A variant feature of marbledom—the color orange—is given an arbitrary 66 to 34 selection advantage during cycles of persistence (i.e. reproduction). Although this advantage builds upon itself through successive generations, an interesting phenomenon occurs, as pointed out by Coen. As the advantaged orange marbles approach saturation in the population, their rate of increase slows. Coen represents this by the prototypical 'S' curve where the rate of convergence is limited by there being fewer blue marbles to select and eliminate.

With the inclusion of reinforcement, the necessary principles of natural selection are in place. A picture emerges in which certain traits that give individual members competitive advantage are persisted through successive cycles of replication. This bias creates a reinforcement that shifts the population toward a change where one trait becomes dominant and another is eliminated. Hence, natural selection, as Coen illustrates, becomes a key driver in the process of evolution.

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