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Is evolution a gradual process or a punctuated one, with rapid evolution taking hold in fits and starts?
A new study suggests the latter, as researchers analyzed the genomes of annelids, like earthworms, and found that these creatures experienced rapid evolution when they transitioned to land hundreds of millions of years ago.
This supports the idea—first proposed in the early 1970s—of “punctuated equilibrium,” where evolution kicks in rapidly after a period of relative genetic stability.
The famed naturalist and geologist Charles Darwin is, of course, best known for his works on evolution—specifically On the Origin of Species and the Descent of Man. However, one year before his death, he wrote a little-known work excellently titled The Formation of Vegetable Mould, Through the Action of Worms, which detailed the important contributions of the world’s “lowly” creatures.
“It may be doubted,” Darwin wrote, “if there are any other animals which have played such an important part in the history of the world as these lowly organized creatures.”
Now, worms are poised to change the world again—this time in a way that even Darwin couldn’t foresee. In a new study published in the journal Nature Ecology & Evolution, scientists from the Spanish National Research Council (CSIC) and Pompeu Fabra University (UPF) in Barcelona, Spain, sequenced the genomes of various earthworms. Upon doing so, they found that these species (in the phylum Annelida) didn’t quite follow Darwin’s ideas of evolution, in which change was a gradual process that played out relatively consistently over time. Instead, they followed an evolutionary path first explored in the 1970s called “punctuated equilibrium.”
The idea is simple. According to Darwin’s theory, the fossil record should be filled with “missing link” species that display minute differences over time. But instead, what we see in our geology appears to be whole missing chapters of a genetic mutation. In an attempt to explain this, punctuated equilibrium posits that rapid evolution can occur periodically after millions of years of relative genetic stability. This would explain why we don’t see as many “missing links” as we might expect.
By synthesizing the complete genomes of these earthworms and comparing them to other annelid species (such as bristle worms and leeches), the researchers were able to travel back some 200 million years and find one such episode of rapid genetic evolution—when these ancient animals transitioned to living on land.
“This is an essential episode in the evolution of life on our planet, given that many species, such as worms and vertebrates, which had been living in the ocean, now ventured onto land for the first time,” Rosa Fernández, lead researcher at the Institute of Evolutionary Biology (part of the CSIC) and senior author of the study, said in a press statement. “The enormous reorganization of the genomes we observed in the worms as they moved from the ocean to land cannot be explained with the parsimonious mechanism Darwin proposed.”
By analyzing the genetic changes happening in this species during this period of rapid evolution, the scientists confirmed that these marine annelids experienced a top-down reorganization of their genetic structure, leaving them unrecognizable. Turns out the humble worm has more to teach us than we thought—though, that likely wouldn’t surprise Darwin in the least bit.
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