Cycles of external dependency drive evolution of avian carotenoid networks

All organisms depend on input of exogenous compounds that cannot be internally produced. Gain and loss of such dependencies structure ecological communities and drive species’ evolution, yet the evolution of mechanisms that accommodate these variable dependencies remain elusive. Here, we show that historical cycles of gains and losses of external dependencies in avian carotenoid-producing networks are linked to their evolutionary diversification. This occurs because internalization of metabolic controls—produced when gains in redundancy of dietary inputs coincide with increased branching of their derived products—enables rapid and sustainable exploration of an existing network by shielding it from environmental fluctuations in inputs. Correspondingly, loss of internal controls constrains evolution to the rate of the gains and losses of dietary precursors. Because internalization of a network’s controls necessarily bridges diet-specific enzymatic modules within a network, it structurally links local adaptation and continuous evolution even for traits fully dependent on contingent external inputs. The mechanisms that accommodate variable external dependencies in evolution are not clear. Here, the authors show that switches between external and internal metabolic controls of carotenoid-producing networks in birds are linked to shifts in evolutionary rates, with internalization of control resulting in bursts of evolutionary diversification.