Evolution in Small Spaces

Photo by Mark Urban.

When Mark Urban noticed that some vernal pools contained an abundance of certain salamanders but the same species was scarce or absent in other nearby pools, he wondered why. What the University of Connecticut researcher found was quite surprising. The difference in the abundance had little to do with the environmental conditions in the various ponds and a great deal to do with the fact that salamanders in different pools had evolved distinct strategies to avoid predation.

Over a 10-week period in 2004, and again in 2010, Urban studied spotted salamander larvae and their numerous predators, including the larvae of marbled salamanders, diving beetles, newts, and dragonflies. He observed that the spotted salamanders behaved differently depending on the particular predators in their environment. For instance, he found that when spotted salamanders lived in the same pools with the larger marbled salamander, the smaller species ate voraciously in an effort to quickly grow too large for the marbled salamander to consume. In pools where marbled salamanders were absent and where diving beetle or dragonfly larvae were in residence, the spotted salamanders ate slowly and discreetly.

“Diving beetles and dragonflies don’t care how big you are. They’re going to eat you anyway,” said Urban. “So spotted salamanders are more cautious when they’re eating in the presence of those predators. Only in ponds where marbled salamanders are found do spotted salamanders eat faster, even though if you’re foraging a lot, you’re going to create a situation where predators see you and eat you. It means you’ll have a low survival rate initially but a high survival rate once you reach the right size.”

Urban also noticed that the size of the spotted salamander tail muscles varied depending on the type of predator living in its pool. By examining the size of the tail muscle relative to the rest of the animal’s body, Urban found that salamanders in ponds that contained an abundance of diving beetle larvae had larger tail muscles.

“Diving beetles don’t seem to give up when chasing their prey, unlike the other predators that appear to give up rather easily,” Urban said. “Salamanders with bigger tail muscles did better at escaping from the beetle larvae than those with smaller tails.”

Perhaps most surprising was that the salamanders and predators he studied were collected from ponds that were very close together. The salamanders could easily have moved from one pond to another yet still evolved different strategies for adapting to the predators they encountered.

“People often assume that evolution occurs over large spatial scales and that you would only find differences in widely scattered populations. But this research showed that we can see adaptations over very limited distances,” Urban said. “It probably means that these animals have a lot of site fidelity, and that they aren’t moving a lot between ponds. Where you grew up is where your kids are probably going to grow up, too. It also suggests that adaptation is much more important than we thought.”

 
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