Simple biological controllers drive the evolution of soft modes

Biological systems respond to environmental challenges through surprisingly simple control mechanisms despite their enormous complexity, yet how low-dimensional controllers can regulate high-dimensional networks remains unclear. We develop a theoretical framework showing selection for environmental robustness drives evolution of “soft modes” in biological state space which enable simple controllers to function effectively. Our model predicts soft-mode mediated environmental stress-response systems also buffer mutations, confirmed using fitness data from yeast strains subject to environmental and mutational perturbations. We also validate the counterintuitive prediction that eliminating controllers reduces stress response dimensionality. This work helps explain the ubiquity of low-dimensional control observed across biological systems and provides testable insights for understanding robustness, with implications for evolutionary biology and the molecular biology of stress response.

Biological systems, with many interacting components, face high-dimensional environmental fluctuations, ranging from diverse nutrient deprivations to toxins, drugs, and physical stresses. Yet, many biological control mechanisms are “simple,” i.e., restoring homeostasis through low-dimensional representations of the system’s high-dimensional state. How do low-dimensional controllers maintain homeostasis in high-dimensional systems? We develop an analytically tractable model of integral feedback for complex systems in fluctuating environments. We find that selection for homeostasis leads to the emergence of a soft mode that provides the dimensionality reduction required for the functioning of simple controllers. Our theory predicts that simple controllers that buffer environmental perturbations (e.g., stress response pathways) will also buffer mutational perturbation, an equivalence we test using experimental data across 5,000 strains in the yeast knockout collection. We also predict, counterintuitively, that knocking out a simple controller will decrease the dimensionality of the response to environmental change; we outline transcriptomics tests to validate this. Our work suggests an evolutionary origin of soft modes, with implications ranging from cryptic genetic variation to global epistasis.