A genetic analysis of Darwin wasps suggests evolution can arrive at similar strategies more than once while making it difficult for lineages to return to earlier ways of life. A Nov. 6 Phys.org report, whose byline names Gesine Steiner and the Natural History Museum, Berlin, summarizes the study, published in Current Biology.
The wasps, members of the family Ichneumonidae, are parasitoids: they lay eggs on or inside living hosts such as caterpillars, beetles and spiders. Their larvae feed on those hosts and eventually kill them. The report says the family includes more than 25,000 described species. Its common name, “Darwin wasps,” was inspired by Charles Darwin’s 1860 letter to botanist Asa Gray, in which he questioned how a benevolent God could create creatures that feed inside living caterpillars.
The study examined genetic data from 697 wasp species representing every major lineage in the group. From those data, researchers reconstructed the evolutionary history of two aspects of parasitoid strategy. Some species paralyze a host before laying eggs, leaving it as an immobile food source. Others let the host continue to live, feed and grow while the wasp larvae develop.
The report says the transition from paralyzing a host to allowing it to keep developing occurred independently at least 23 times. The researchers found no evidence that lineages made the reverse transition. In other words, similar strategies arose repeatedly across the wasps’ evolutionary history, but the analysis did not identify a return to the ancestral strategy.
A second pattern involved where larvae develop. Larvae developing inside rather than outside a host also evolved repeatedly, the report says, but this change was only rarely reversed. The two results therefore differ in strength: the analysis found no reversals for the host-paralyzing strategy shift, while reversals in the inside-versus-outside pattern were rare.
The researchers were led by the Museum für Naturkunde Berlin. The report identifies Bernardo F. Santos, a researcher at the museum, as the study’s lead author. Santos describes the pattern as an evolutionary ratchet: once a lineage becomes closely tied to a living, developing host, returning to its earlier strategy may become extremely difficult.
That interpretation speaks to a central question in evolution: how predictable are evolutionary changes, and how much do earlier adaptations constrain what can happen next? The reported pattern combines repeatability with directionality. Different lineages arrived at similar ways of interacting with hosts, yet their histories appear to have narrowed the routes back. Evolution, in this account, does not explore every option equally freely; changes made along the way can make some alternatives harder to reach.
The findings concern evolutionary histories reconstructed from genetic data. “No evidence of reversals” does not establish that reversal is impossible in every lineage, and the report characterizes the second pattern as rarely reversed. Together, the results support the idea that evolutionary history can constrain future change, while leaving the precise degree of constraint dependent on the strategy being examined.