Daphnia Pulex, a.k.a. the water flea – a widely studied microscopic crustacean that’s so ancient it has survived the past 500 million years with very few changes – is pointing out an entirely different hypothesis on how quickly species can evolve. According to recent research, this small species is now evolving faster and more complexly than scientists had believed possible.
A Decade-Long Genetic Investigation
In a study published in the July 2024 issue of the Proceedings of the National Academy of Sciences (PNAS), Michael Lynch of Arizona State University and colleagues spent 10 years sequencing the genomes of nearly 1,000 individuals belonging to Daphnia pulex. They followed genetic diversity over time in a naturally isolated water flea population using next-generation genomics techniques.
What they found was a surprise: In an environment that seemed stable and had no obvious external driving factors, the frequencies of different alleles were changing from one year to the next — even for alleles exhibiting relatively elevated selective advantage. The fact that this genetic “churn” is constant implies that the process of natural selection may work differently than traditional evolution models suggest.
The Unseen Complexity of Evolution
In the past, it was thought that if no changes occur in the environment, its genetic structure has also to remain fixed. But that is not what the water flea genome says. The study found that while the overall strength of natural selection averaged near zero over the decade, there were substantial temporal changes in allele frequencies at specific chromosomal regions.
“Even without external environmental pressures, there’s a lot happening at the genetic level that we’re just beginning to comprehend.” – explains Michael Lynch
Implications for Adaptation and Genetic Diversity
This research, therefore, arguably has implications for how species maintain key genetic diversity required to adapt to environmental change in the future. This variation in the frequencies of alleles means that each population has a pool of genetic variants sitting ready for use if circumstances alter.
Lynch also found evidence that genes located near each other on a chromosome evolved together. This genetic linkage allows for the inheritance of beneficial allelic combinations, which can speed up the adaptation process. But that association might also pull less beneficial alleles along with it, and in doing so, reduce the overall efficacy of natural selection.
A New Lens on Evolutionary Biology
The results of the Daphnia pulex study should be a call to reconsider how genetic diversity and natural selection are interpreted in wild populations by scientists. Genetic divergence is only one aspect of evolutionary change and reflects change leading to reproductive isolation, so traditional methods for assessing germination from genetic distance may not provide full insight into the more subtle and ongoing evolutionary processes underlying present-day conservation challenges.
“This research opens up new avenues for understanding the mechanisms of evolution,” says a collaborator from Central China Normal University. “It underscores the importance of long-term, genome-wide studies to truly grasp how species adapt over time.”
Broader Ecological Significance
The study looks at a tiny organism, but its findings have far-reaching implications. Insights into this fast and intricate evolutionary process of Daphnia pulex may enable us to predict the responses of other species, including how they cope with environmental stressors such as pollution and global climate change. Because water fleas are a key part of freshwater ecosystems (being one of the major food sources for fish and controlling algal populations) their rapid adaptation has large affects down the rest of the food web.
Rethinking Evolution in a Changing World
The sheer fact that Daphnia Pulex is adapting so rapidly, even in a stable environment, demonstrates us the complexity and flexibility of life. This work underscores the need for scientists to be aware of temporally-varying genetic structure in evolutionary studies and that evolution is not necessarily a slow, gradual process.
Credits: Arizona State University