Two opposing evolutionary forces explain the presence of two different colors of spotted salamander egg masses in ponds in Pennsylvania, according to a new study led by a Penn State biologist. Understanding the processes that support the biological diversity of wild populations is a central issue in biology and may allow researchers to predict how species will respond to global changes.
Spotted salamanders (Ambystoma maculatum) are a widespread species that occur in the eastern United States and return to temporary ponds in the spring to reproduce. Female salamanders lay their eggs in clusters called egg masses, which are either opaque white or perfectly clear. Females lay masses of eggs of the same color throughout their lives, but it is unclear what causes the different coloration, or if either of these colors gives an advantage to the eggs, such as if one color is less obvious to predators.
"We usually think of evolution acting over hundreds or thousands of years, but in reality, the evolutionary processes in a system can affect every generation of animals," said Sean Geary, an Eberly postdoctoral research fellow at Penn State and the leader of the research team. "In this study, we examined ponds that were originally studied in the early 1990s, which gave us a unique opportunity to study the evolutionary processes that shape the frequencies of the two types of egg mass color, or morphs, that we see today."
Geary reinsured a network of 31 ponds in central Pennsylvania by identifying the color of the salamander egg masses, as well as the environmental characteristics of each pond. The ponds were originally surveyed in 1990 and 1991 by then-Penn State biology professor Bill Danson and his students. The new study is published on April 14 in the journal Biology Letters.
The research team found that the salamander population size and pond chemistry have remained stable over the past three decades. With an average across the region, the overall frequency of each egg color morph also remained the same - about 70% of white egg masses in 1990 and 2020-but in many cases the frequency in individual ponds changed dramatically.
"On the scale of individual ponds, this is an extremely dynamic system," Geary said. "They don't just reach one frequency and stay there. By focusing on individual ponds, rather than just the region as a whole, we could tease apart what is driving these changes in population frequencies. In this case, we found two opposite evolutionary processes – selection and drift."
The researchers found strong signatures of an evolutionary process called genetic drift, which can lead to changes in morphing frequencies due to randomness. In small populations, drift is likely to have a serious effect, such as when one of the morphs disappears completely. As expected due to the drift, the researchers found that the frequencies of each morphing changed more dramatically in ponds with fewer egg masses.
"However, none of the ponds have completely switched to one morphing or the other, which suggests that something else may also be going on," Geary said. "We found that ponds at extremes in the 1990s - with a high frequency of clear or high frequency of white egg masses-became less extreme, shifting towards the overall average for the region. This supports the idea that there is a "balancing selection" in this system.
Selection balancing is a type of natural selection that can help preserve multiple traits or morphing in a population. According to Geary, one possible explanation for balancing selection in the color of the egg mass is that rare morphing in the pond, regardless of the actual color, has an advantage that will lead to rare morphing becoming more common. Another possibility is that white morphing has an advantage in some ponds while clear morphing has an advantage in others, and the movement of the salamander between the ponds results in the persistence of both morphs.
"Ultimately, we found a tension between these two evolutionary processes, with genetic drift potentially leading to a reduction in diversity in this system, and balancing selection works to maintain it," Geary said.
Researchers are currently surveying egg masses in ponds outside of Pennsylvania to study if morph frequencies differ in other regions and whether these evolutionary processes work in the same way on a larger scale.
"Although we did not see a relationship between egg mass color and environmental characteristics in this study, it is possible that environmental characteristics on a larger scale may lead to an optimal frequency for each region," Geary said. "By looking at a much broader scale, we can get a better idea of whether regional optimal conditions exist and how they are maintained. Understanding the processes that support biological diversity may ultimately help us predict how wild animals adapt to our changing world."