Law as Constructed Niche: Reciprocal Causation Between Extended Phenotypes and Selective Environments in Institutional Evolution

Institutional evolutionary theory faces an unresolved causal direction problem. Extended Phenotype Theory (EPT) treats legal institutions as downstream phenotypic expressions of competing memes, while Niche Construction Theory (NCT) proposes that organisms actively modify their selective environments, generating reciprocal causation that EPT's unidirectional framework cannot capture. This paper argues that legal institutions are simultaneously (a) extended phenotypes of competing memes and (b) constructed niches that modify the selection pressures acting on those same memes and on future institutional generations. The synthesis integrates EPT (Dawkins 1982; Lerer 2025a, 2025b, 2025c) with NCT (Odling-Smee, Laland and Feldman 2003; Laland et al. 2015), engages the Ecological Niche of Knowledge framework (Khurshid 2025a, 2026a-f) as a case of civilizational-scale niche construction, and incorporates three contributions from Gould: exaptation as unintended niche construction, multilevel selection as the framework for cross-scale reciprocal coinfluence, and punctuated equilibrium as the macroevolutionary model for institutional stasis and crisis-driven change. Using the Constitutional Lock-in Index as a measure of niche rigidity and Heteronomous Bayesian Updating as the micro-level learning mechanism, six falsable predictions are derived: (P1) institutions with strong niche feedback will show different evolutionary trajectories; (P2) legal transplants will fail more often when the ecological inheritance gap is large; (P3) niche-modification reforms will outperform phenotype-attack reforms in high-CLI systems; (P4) high-CLI systems will exhibit niche conservatism beyond ESS predictions; (P5) ENK thickness will correlate with CLI and IEI; and (P6) institutional exaptations will show a distinctive stasis-punctuation signature distinguishable from intentionally designed niches. Implications follow for EPT, for Tgmenks, for computational law, and for institutional design.