Natural selection and the Michaelis constant

Natural selection operating on a biological process with Michaelis-Menten kinetics adjusts the Michaelis (half-saturation) constant relative to ambient substrate concentration. From the perspective developed here, the relative fitnesses of alternative evolutionary “strategies” determine the trajectory of the Michaelis constant (K_m) over evolutionary time. If substrate concentration is held relatively constant or fluctuates randomly about mean concentration over evolutionary time, while processing rate increases, K_m tends toward a value greater than or equal to substrate concentration; if processing rate is held relatively constant over evolutionary time, K_m tends to become large relative to substrate concentration. The diversity of the supportive evidence cited suggests a broad applicability of this argument across taxonomic groups and levels of biological organization.