Carnivorous plant traps help scientists explain the evolution of complex ‘composite’ traits

An international team of researchers led by Dr Ulrike Bauer from the University of Bristol studied two species of tropical pitcher plants separated by 4,000 km across: the slender pitcher plant (Nepenthes gracilis) found in Borneo and Nepenthes pervillei from the Seychelles islands in wide. the east coast of Africa.

Dr Bauer and his team found that the two plants had independently evolved the same unusual “stepping stone” mechanism for capturing insects. When raindrops fall on the pitcher plant’s roof-shaped trap lid, insects under the trap lid are catapulted into the trap below. The work is published in the journal Science.

This springboard trapping mechanism only works if three independent traits come together. First, the lid must be horizontal so that the catapulted prey lands inside the trap. Second, the cover must act as a spring so that the impact energy of the raindrop is transferred to the insect. Finally, the underside of the lid is covered with a layer of fine wax crystals that give it the ideal level of slipperiness: grippy enough that insects can walk upside down under the stationary lid, but slippery enough that they lose their grip in the event of a fall. shots.

All three components are necessary for the trap to work. Springboard trapping is what Dr. Bauer calls a “composite trait.”

Evolutionary biologists struggle to explain the emergence of such composite traits. “We learn in school that evolution works by natural selection acting on naturally variable traits,” says Dr. Bauer. Individuals with advantageous traits have more offspring, and so over time the frequency of genes underlying the advantageous traits increases in the population. »

“This simple concept explains how giraffes’ necks became longer and cheetahs’ necks became faster over time, but it struggles to explain the emergence of entirely new traits, particularly when these come from ‘a combination of multiple, previously unrelated components, such as the stepping stone mechanism of the pitcher plant.’.

Dr. Bauer and his team developed an alternative hypothesis for the evolution of composite traits: spontaneous coincidence, or in other words, the chance encounter of a new beneficial combination.

“Our research has shown that in the majority of Nepenthes plant species, all three features of the stepping stone trapping mechanism are unusually variable,” Dr Bauer said.

“This increased the pool of possible trait combinations, and thus the probability of a new beneficial combination. In this case, it is actually the absence of strong selection that facilitates evolution, because it allows traits to “be more variable, thus increasing the number of characters. of the “tickets” in the evolutionary lottery.”