Science & Technology

Competing evolutionary forces drive the rapid evolution of salamander colors

Spotted salamander is a widely distributed species in the eastern United States that breeds back in temporary ponds in the spring.Credits: Sean Giery, PA

Two opposing evolutionary forces explain the existence of two differently colored spotted salamander eggs in a pond in Pennsylvania, according to a new study led by a biologist at Pennsylvania State University. Understanding the process of maintaining biodiversity in wild populations is a central issue in biology, allowing researchers to predict how species will respond to global changes. I will.

Spotted salamander (Ambystoma maculatum) is a widespread species that occurs in the eastern United States and breeds back in temporary ponds in the spring. Female salamanders lay their eggs in opaque white or completely transparent masses called egg masses. Females lay eggs of the same color throughout their lives, but it is unclear what causes different colors or whether any of these colors will benefit the eggs. For example, one color may not be very noticeable to a predator.

“We usually think that evolution has been functioning for hundreds or thousands of years, but in reality, the evolutionary process taking place in the system can affect animals of each generation. It’s possible, “said Sean Jary, a postdoctoral fellow and research leader at Penn State University. team. “This study reviewed the ponds that were first studied in the early 1990s, giving us a unique opportunity to explore the evolutionary processes that shape the color types or morph frequencies of the two egg masses we see today. Was done. “

Giery reviewed the network of 31 ponds in central Pennsylvania, focusing on salamander egg mass color and environmental characteristics of each pond. The pond was originally investigated in 1990 and 1991 by Penn State University Professor of Biology Bildanson and his students.The new study will be published in the journal on April 14, 2021 Biology letter..

Spotted salamander egg mass

Two opposing evolutionary forces help explain why two differently colored spotted salamander eggs are found in ponds in central Pennsylvania, according to a new study. The egg mass is completely transparent or opaque white.Credit: University of Connecticut Mark Urban

The researchers found that salamander populations and pond chemistry have been stable for the past 30 years. On average across the region, the overall frequency of color change for each egg remained the same (approximately 70% egg white mass in both 1990 and 2020), but often in individual ponds. Has changed significantly.

“At the scale of the individual ponds, it’s a very dynamic system,” says Giery. “They don’t just reach and stay there at one frequency. By focusing on individual ponds rather than the entire region, we can isolate what is causing these changes in population frequency. In this case, we found two conflicting evolutionary processes: selection and drift. “

Researchers have discovered a strong sign of evolution called genetic drift. This can change the frequency of morphs by chance. If the population is small, drift is likely to have a significant impact. For example, one of the morphs disappears altogether. As expected due to drift, researchers found that the frequency of each morph changed more dramatically in ponds with low egg yields.

“But none of the ponds have completely migrated to one of the morphs, which suggests that something else may be happening,” Giery said. “We found that the extreme ponds of the 1990s (high frequency of clear egg masses or high frequency of white egg masses) are no longer extreme and are shifting to the overall average of the region. This supports the idea that “balanced selection” works in this system. “

Balancing selection is a type of natural selection that helps maintain multiple properties or morphologies within a population. According to Giery, one possible explanation for balancing the choice of egg mass color is that the rare morphs in the pond have the advantage of becoming more common, regardless of the actual color. is. Another possibility is that white morphs are advantageous in some ponds, while transparent morphs are advantageous in other ponds, and the movement of salamanders between the ponds leads to the persistence of both morphs. ..

“Ultimately, genetic drift can lead to a reduction in the diversity of this system, and balancing the choices that work to maintain it has found tension between these two evolutionary processes. “Giery said.

Researchers are currently investigating egg masses in ponds outside Pennsylvania to see if morph frequencies differ in other regions and if these evolutionary processes work in the same way on a larger scale. I will.

“This study did not find a relationship between egg mass color and environmental characteristics, but larger environmental characteristics may promote optimal frequency in each region,” Giery said. .. “Looking at a much larger scale gives us better ideas about whether there are regional optimizations and how they are maintained. The process of maintaining biodiversity. Understanding can ultimately help predict how wildlife adapts in a changing world. “

Reference: April 14, 2021 Biology letter..
DOI: 10.1098 / rsbl.2020.0901

In addition to Giery, the research team includes Marketa Zimova at the University of Michigan and Dana Drake and Mark Urban at the University of Connecticut. This work was supported by the National Science Foundation, the University of Michigan Institute for Global Change Biology, and Penn State University of Everly Science.

Competing evolutionary forces drive the rapid evolution of salamander colors Competing evolutionary forces drive the rapid evolution of salamander colors

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