Evolving Darwin's ‘most wonderful’ plant: ecological steps to a snap‐trap

Summary Among carnivorous plants, Darwin was particularly fascinated by the speed and sensitivity of snap‐traps in Dionaea and Aldrovanda . Recent molecular work confirms Darwin's conjecture that these monotypic taxa are sister to Drosera , meaning that snap‐traps evolved from a ‘flypaper’ trap. Transitions include tentacles being modified into trigger hairs and marginal ‘teeth’, the loss of sticky tentacles, depressed digestive glands, and rapid leaf movement. Pre‐adaptations are known for all these traits in Drosera yet snap‐traps only evolved once. We hypothesize that selection to catch and retain large insects favored the evolution of elongate leaves and snap‐tentacles in Drosera and snap‐traps. Although sticky traps efficiently capture small prey, they allow larger prey to escape and may lose nutrients. Dionaea 's snap‐trap efficiently captures and processes larger prey providing higher, but variable, rewards. We develop a size‐selective model and parametrize it with field data to demonstrate how selection to capture larger prey strongly favors snap‐traps. As prey become larger, they also become rarer and gain the power to rip leaves, causing returns to larger snap‐traps to plateau. We propose testing these hypotheses with specific field data and Darwin‐like experiments. The complexity of snap‐traps, competition with pitfall traps, and their association with ephemeral habitats all help to explain why this curious adaptation only evolved once.