Evolution can repeat itself, says Utah State University alum Samridhi Chaturvedi PhD’19, resulting in parallel adaptations in independent lineages occupying similar environments. Consider the plant-eating stick insect Timema: Multiple species of the genus live throughout California, sporting varied colors enabling the wingless creatures to blend in with their varied habitats.
“We might surmise these different colors, which allow the insects to avoid detection by predators, provide evidence of evolution by natural selection,” says Chaturvedi, a postdoctoral fellow at the University of California, Berkeley. “On the other hand, idiosyncratic evolutionary outcomes — like the loss of a color pattern in one species — can be driven by chance or contingency, which indicate constraints on the power of natural selection in driving evolution.”
Chaturvedi, her former USU faculty mentor Zachariah Gompert of the Department of Biology and the Ecology Center, along with colleagues at the University of Notre Dame; Bangor University, Royal Holloway University and the Jones Ines Center of the United Kingdom; Switzerland’s University of Bern, and the CEFE research center at the University of Montpellier in France, published findings about these questions in the Oct. 24, 2022 issue of Nature Ecology & Evolution. Chaturvedi’s research was supported by the USU College of Science, the USU School of Graduate Studies and the National Science Foundation.
“Several studies have indicated striking parallel evolution, which is often driven by parallel changes at single genes of major effect,” says Chaturvedi, who will join Tulane University’s faculty as an assistant professor in fall 2023.
These genetic regions were also enriched for genes showing rapid change during a field experiment and genes associated with a trait that prevents desiccation in insects, she says, which support a causal connection to climate adaptation in these species.
“We explored three hypotheses,” Chaturvedi says. “We asked whether similarity in the climatic variation was experienced by different species — a shared ecology hypothesis — whether the similarity resulted from the genome composition of species — a shared genetics hypothesis — or whether both of these factors best explained the degree of parallelism in genotype-climate associations across species.”
She and fellow researchers investigated the genomic basis of parallel adaptation to climate, including changes in temperature and precipitation, in eight species of Timema.
“We first identified regions of the genome associated with climate within each of the eight species, which show a range of divergence times of up to tens of millions of years,” Chaturvedi says. “We assessed the contributions of these factors to genomic parallelism by comparing the proportions of the genome that exhibit repeated genotype-climate association.”
The researchers identified decay in parallelism with both increased ecological (climatic) and genetic distance for a given pair of species, supporting both the shared ecology and shared genetics hypotheses.
“Our findings shed light on when evolution is most expected to repeat itself,” Chaturvedi says. “Our study shows that both similar ecological conditions, which drive natural selection, and similar pools of genetic variation, which provide the material for evolution, are critical for generating repeated evolutionary outcomes.”
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