Computational models of physics suggest reality operates via a fundamental update rule, yet they lack an explicit physical mechanism for its instantiation. Constructor theory mandates that any reliable transformation requires a physical constructor. This paper resolves this gap by proposing the 'feme' — a fundamental replicating information structure — as the necessary constructor.We frame the fundamental update rules, such as those described in Wolfram Physics or supersymmetry, as the transformations caused by the interaction of the feme with reality. We argue that femes, like genes, memes, and temes, are replicators that embody knowledge for creating both a phenotype and a copy of itself. Their evolution by natural selection provides a causal mechanism for the stability and fine-tuning of physical laws, predicting observable signatures including a self-replication program, embedded error-correcting codes, and the fallibility of physical laws.