Researchers at the Museum für Naturkunde Berlin analyzed genetic data from 697 Darwin wasp species to understand whether evolution can reverse course. They reconstructed the evolutionary history of parasitic strategies in these insects, which lay eggs on or inside living hosts like caterpillars, beetles, and spiders.
The team discovered that the shift from paralyzing hosts to allowing them to continue developing occurred independently at least 23 times across different wasp lineages. However, they found no evidence that any lineage reversed this change. Similarly, the evolution of larvae developing inside rather than outside hosts happened repeatedly but was only rarely reversed.
"Evolution doesn't necessarily keep all its options open," says Bernardo F. Santos, lead author and researcher at the Museum für Naturkunde Berlin. "Once a wasp lineage evolves a close relationship with a living, developing host, returning to its ancestral strategy seems extremely difficult. It's something like an evolutionary ratchet: you can move forward, but going backward becomes nearly impossible."
The results appear in Current Biology and show that evolution can be both highly repeatable across independent lineages and strongly directional. Once organisms evolve particular adaptations, the constraints of their evolutionary history make reversions nearly impossible, even when ancestral strategies might theoretically be viable.