"(...)Another study, this time appearing in the New Phytologist, presented evidence for the evolution of snap traps of Dionaea and Aldrovanda from a flypaper trap like D. regia, based on molecular data. The molecular and physiological data implies that Dionaea and Aldrovanda snap traps evolved from the flypaper traps of a common ancestor with the Drosera; the living evidence of a link between Drosera and Dionaea is D.*regia and its remnant characteristics.
In this evolutionary model, pre-adaptations to evolution into snap-traps were identified in several species of Drosera, such as rapid leaf and tentacle movement. The model proposes that plant carnivory by snap-trap evolved from the flypaper traps driven by increasing prey size.
Bigger prey provides increasingly higher nutritional value, but large insects can easily escape the sticky mucilage of flypaper traps; the evolution of snap-traps would prevent escape and kleptoparasitism (theft of prey captured by the plant before it can derive any benefit from it), and would also permit a more complete digestion.
Larger insects usually walk over the plant, instead of flying to it, and are more likely to break free from sticky glands alone. Therefore, a plant with wider leaves must have adapted to move the trap and it's stalks in directions that maximized its chance of capturing and retaining such prey - in this particular case, longitudinally. Once adequately "wrapped", escape would be more difficult.
Then, evolutionary pressure selected the plants with shorter response time, in a manner similar to Drosera burmannii or Drosera glanduligera. The faster the closing, less reliant on the flypaper model the plant would be.
As the trap became more and more active, the energy demanded to "wrap" the prey increased. Therefore, plants that could somehow differentiate between actual insects and random detritus/rain droplets would be in advantage, thus explaining the specialization of inner tentacles into trigger hairs.
Ultimately, as the plant relied more in closing around the insect rather than gluing them, the tentacles so evident in Drosera would lose its original function altogether, becoming the "teeth" and trigger hairs — an example of natural selection hijacking pre-existing structures for new functions.
Completing the transition, at some point in its evolutionary history the plant developed the depressed digestive glands found inside the trap, rather than using the dews in the stalks, further differentiating it from the Drosera genus.(...)"