First comes some gene A which is simple, but at least a little useful on its own, so that A increases to universality in the gene pool. Now along comes gene B, which is only useful in the presence of A, but A is reliably present in the gene pool, so there’s a reliable selection pressure in favor of B. Now a modified version of A* arises, which depends on B, but doesn’t break B’s dependency on A∕A*. Then along comes C, which depends on A* and B, and B*, which depends on A* and C. Soon you’ve got “irreducibly complex” machinery that breaks if you take out any single piece.
Gene pool
