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A type of zooplankton, Daphnia pulex (a.k.a. the water flea) is a microscopic crustacean whose unique reproduction habits make it the perfect species for studying evolution and natural selection.
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A new study looks at how natural selection works more subtly than previously believed while an organism is in a stable environment.
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Insights into this evolutionary mechanism could help scientists understand the adaptive capacity of organisms in the wild while also developing strategies to harden vulnerable food webs against climate change.
Daphnia pulex, known more commonly as the water flea, is barely perceptible to the naked eye but plays a vital role in freshwater ecosystems around the world as prey to both vertebrate and invertebrate species. Water fleas also keep algae blooms (which can cause millions of dollars in damage to the U.S. economy) in check. But outside its watery habitat, this minuscule crustacean similarly leaves an outsized legacy in the world of science.
That’s because D. pulex is what’s known as a “model organism,” a species that’s been extensively studied to understand particular biological phenomena. The water flea was the first crustacean to have its genome sequenced back in 2011 (by the way, the organism has more genes that us humans), and the creature has been crucial to biologists’ ongoing mission to understand the mechanisms of evolution. Because of their clonal, or asexual, reproduction, scientists have studied genetic variation among traits, including age, birth size, growth rate, immune response and much more.
Now, scientists from Arizona State University, along with colleagues from Central China Normal University and Notre Dame, have analyzed nearly 1,000 D. pulex specimens and discovered the strength of natural selection on individual genes year by year. Surprisingly, the team found significant fluctuation in gene variants, known as alleles, even in stable conditions, a strategy that could possibly increase the ability to adapt to changing environments while also maintaining genetic variation. The results of the study were published in the journal Proceedings of the National Academy of Sciences (PNAS).
“This study has, for the first time, given us a glimpse into the kinds of temporal changes in gene frequencies that occur even in seemingly constant environments,” Arizona State University’s Michael Lynch, lead author of the new study, said in a press statement, “a sort of ongoing churn of genetic variation distributed across the genome.”
Because of Daphnia’s rapid reproduction cycle and sensitivity to environmental pollutants, members of the genus are often used to test freshwater toxicity, but in the laboratory, those traits have become a critical tool for studying evolution. Across a 10-year span, the study analyzed the genetic variance of D. pulex in a stable environment. The study showed that the organisms experienced changing selection pressures, but that they all eventually canceled out, meaning no dominant trait took over and influenced the organism’s evolution. In other words, this shows that evolution works on a level much more subtle than what scientists previously believed.
While this challenges the typical notion of studying genetic diversity and divergence as the only pieces of evidence for the intensity of selection pressures, this biological strategy also helps the organism retain genetic diversity while also preparing for rapid adaptation. For example, the scientists note that genes located on chromosomes near each other evolved in coordination with each other. This could cause beneficial combinations of gene variants to be inherited, thereby speeding up adaptation to the environment around them.
Of course, in an era of climate change, understanding these underlying evolutionary mechanisms is important. Because it’s impossible to perfectly recreate the evolutionary pressures on species in the wild, understanding the baseline functions on a species in a stable environment can help form a foundation for how other species could be well-suited for adaptation. Eventually, this could help scientists build resiliency in certain populations to protect vitally important food webs.
Science is delving deeper into the wondrous world of evolution, and it’s all thanks to a microscopic crustacean.
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