According to a new study led by biologists at Pennsylvania State University, two opposite evolutionary forces explain the presence of two different colors of Ambystoma maculatum in Pennsylvania ponds. Understanding the process of maintaining biodiversity in wild populations is a central biological issue that could lead researchers to predict how species will respond to global change.
Ambystoma maculatum is a widely available species that appear in the eastern United States and returns to makeshift ponds in the spring to breed. Female Ambystoma maculatum lays eggs in a block called an egg mass, which is opaque white or completely transparent. Females produce the same colored ovaries throughout their lives, but it is not clear what causes them to change colors.
“We usually think of evolution as operating over hundreds or thousands of years, but in fact, evolutionary processes that work in a system can affect every generation of animals,” said Sean Giery, a postdoctoral fellow at Pennsylvania State University and a member of the research team. “In this study, we re-examined the pond originally studied in the early 1990s, giving us a unique opportunity to explore the evolutionary process of the frequency of the two egg group color types or morphology we see today.”
Giery re-examined a network of 31 ponds in central Pennsylvania and noted the color of Ambystoma maculatum’s ovulation and the environmental characteristics of each pond. The ponds were originally investigated by Bill Dunson, then a professor of biology at Pennsylvania State University, and his students in 1990 and 1991. The new study was published April 14, 2021, in the Biology Letters.
The team found that Ambystoma maculatum’s population size and pond chemistry have remained stable over the past 30 years. When the entire region is averaged, the overall frequency of each ovulation pattern remains the same — about 70 percent of the white ovulations in 1990 and 2020 — but in many cases, the frequency in a single pond changes dramatically.
“It’s an extremely dynamic system in terms of the size of a single pond, ” says Giery. “They don’t just reach a frequency and stay there. By looking at individual ponds, not just the entire region, we can identify what is driving the changes in the frequency of these populations. In this case, we found two opposite evolutionary processes — selection and drift. ”
The researchers found a strong signal of an evolutionary process called gene drift, which can cause morphological frequencies to change by chance. In small populations, drift is more likely to have a significant impact, such as the complete disappearance of one of these patterns. As expected, as a result of drift, the researchers found that in ponds with fewer eggs, the frequency of each form changed more significantly.
“However, no pond has completely shifted to one form or another, suggesting that something else may have happened,” Giery said. “We found that ponds that were extreme in the 1990s — white ovulations with high frequencies of clarity or high frequencies — became less extreme and shifted to the overall average of the region. This supports the idea that ‘balanced selection’ runs in this system. ”
Balanced selection is a type of natural selection that helps preserve multiple traits or patterns in a population. According to Giery, one possible explanation for the balanced selection of egg colors is that rare forms in ponds — regardless of actual color — have an advantage, which leads to rare patterns becoming more common. Another possibility is that white forms have advantages in some ponds, while transparent forms have advantages in other ponds, and the movement of dragonflies between ponds leads to the persistence of both forms.
“Ultimately, we found the tension between these two evolutionary processes, and genetic drift may lead to a reduction in the diversity of the system, while balanced choices are committed to maintaining it,” Giery said.
Researchers are currently investigating egg clusters in ponds outside Pennsylvania to see if the morphological frequencies are different in other regions and whether these evolutionary processes work in the same way on a larger scale.
“While we didn’t see a relationship between egg color and environmental characteristics in this study, a broader range of environmental characteristics may drive optimal frequency for each region,” Giery said. “By looking at larger scales, we can better understand whether there are regional optimal and how to maintain them. Understanding the process of maintaining biodiversity may ultimately help us predict how wildlife will adapt to our changing world. ”