Evolution Repeats Itself: Jellyfish Genomes Reveal Identical Genetic Changes Across 600 Million Years

Evolution has long been imagined as an unpredictable tinkerer, throwing together mutations at random and letting natural selection sort out the winners. But a new study from researchers at the University of California, Santa Barbara, the University of Kansas, Texas A&M University and the University of Nebraska suggests that at the deepest levels of the genetic code, evolution may be far more repeatable than scientists ever suspected. By reconstructing sections of jellyfish genomes and tracing them back through more than half a billion years of history, the team uncovered evidence of identical genetic changes arising independently in multiple lineages separated by more than 600 million years of evolution.

The research, published in the Proceedings of the National Academy of Sciences, focuses on a phenomenon known as adaptive molecular convergence. This is the process by which distantly related, or even entirely unrelated, species evolve the same specific genomic changes. While biologists have documented such convergence before, it has generally been considered a rare curiosity. The new findings upend that assumption, revealing an unexpectedly high number of repeated molecular evolutions scattered across the jellyfish family tree.

Senior author Todd Oakley, a biologist at UC Santa Barbara, explained that the study was driven by one of the most fundamental questions in evolutionary biology: how repeatable is evolution, and under what time scales and circumstances does it repeat itself? Answering that question, he noted, offers a window into the predictability of evolution itself, potentially pointing toward a future in which scientists could anticipate evolutionary outcomes, including how particular genes give rise to particular physical traits.

To probe that question, the researchers turned to Medusozoa, an ancient and extraordinarily diverse lineage that includes jellyfish and their relatives. Emerging roughly 650 million years ago, members of this clade have undergone repeated and convergent phenotypic evolution, including multiple independent origins of eyes and repeated losses of the free-swimming medusa stage. That combination of deep age and known convergent traits made the group an ideal natural laboratory for testing whether genetic changes track physical ones.

The first challenge was reconstructing the evolutionary history of Medusozoa itself. Using both fossil and molecular data, the team built an evolutionary tree and applied a method called ancestral state reconstruction to infer when and where convergent traits, such as eyes, emerged across the lineage. The resulting phylogeny ranks among the most comprehensive jellyfish evolutionary trees assembled to date, providing a robust framework for comparing patterns of change across hundreds of millions of years.

With that tree in hand, the researchers sequenced the genes expressed in the different jellyfish species and examined the patterns of change across those genes. Whenever the same specific genetic change appeared in distantly related species, the team classified it as molecular convergence. What they found surprised them: repeated molecular evolutions, represented by identical genetic changes across different species, occurred far more often than expected, with some instances spanning lineages separated by more than 600 million years of independent evolution.

Yet the most intriguing finding came from what the data did not show. The identical genetic changes were largely unassociated with the jellyfishes’ phenotypic convergence, meaning the evolution of similar physical traits in distantly related species. In other words, jellyfish that independently evolved similar bodies or similar features did not predictably share the same underlying genetic changes. This suggests a far more complex relationship between genetic and phenotypic convergence than the straightforward assumption that look-alike organisms carry look-alike genes.

According to the researchers, the explanation lies in the nature of natural selection itself. While selection can promote the same genetic change in disparate species, molecular convergence appears to be the product of a more intricate process, one that reflects the ways organisms interact with their environments. Genetic similarity, in this view, accumulates through a web of ecological pressures and historical contingencies rather than as a simple one-to-one mapping between genes and physical form.

Cory Berger, a postdoctoral researcher in Oakley’s laboratory and a lead author of the study, emphasized how little is still known about the extent to which similar genetic changes occur in different organisms. The sheer volume of molecular convergence documented in the jellyfish genomes, however, offers a tantalizing clue. As Oakley put it, the amount of genetic similarity indicates that at the genetic level, evolution may be more repeatable, and perhaps therefore more predictable, than scientists expected. At the same time, he cautioned, the findings reveal how much remains to be learned about how genes function and how that genetic play unfolds at the scale of the whole organism.

The implications reach well beyond jellyfish. If identical genetic changes can arise independently across lineages separated by more than 600 million years, then the raw material of evolution may be governed by deeper constraints than previously recognized, constraints that could shape how life responds to environmental change on any time scale. For a field that has often treated evolutionary outcomes as fundamentally contingent, the jellyfish genomes offer a provocative counterpoint: rewind the tape of life, and at least some of the same mutations may play again. The study, titled Adaptive molecular convergence is pervasive across deep time and largely decoupled from phenotypic convergence, stands as a landmark demonstration that evolution’s genetic playbook, whatever its quirks, is more repeatable than anyone imagined.

Subject of Research: Adaptive molecular convergence in Medusozoa genomes across deep evolutionary time

Article Title: Adaptive molecular convergence more common than previously thought

Article References: Adaptive molecular convergence more common than previously thought. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: molecular convergence, jellyfish, Medusozoa, evolutionary biology, genomics, phylogenetics, natural selection, phenotypic convergence, ancestral state reconstruction, PNAS, UC Santa Barbara, deep time