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Humans Driving Evolution of the Spotted Salamander
Environment Yale, July 2012

“There’s some hopping going on here,” says Steve Brady.

Brady is mucking through a pond. His hair, black dusted with gray, swirls over his forehead. He wears hip-high wading boots, which keep him dry as he wobbles in the deep mud and negotiates downed logs. The pond he’s slogging through sits on the eastern edge of Westwoods, a forest preserve in Guilford, Conn. Piles of tumbled granite boulders and stands of maples rise over the water. Long-legged mosquitoes drift around Brady, looking for a patch of skin. Water striders flit along the pond’s surface to get out of his way. A frog croaks from time to time. Brady stops and scans the water like an egret, and then he shoots his arm into the water. He brings up his hand in a delicately clenched fist.

“I’m not sure what I’ve got,” he calls out. “I just see the nose.”

Brady climbs out of the water and up to a bed of moss. He opens up his hand to see what he’s caught. Out squirms a larval spotted salamander. Adult spotted salamanders develop two rows of neon yellow spots running down their dark gray backs. This salamander, smaller than a dragonfly, is blush-colored. It is in that awkward in-between stage of amphibian adolescence: it has just sprouted its legs, but its tail remains more like a fish fin, and external gills still hang from its slimy neck.

“He’s ready to go whenever he wants,” says Brady. Spotted salamanders can time their development to make the best use of their pond. If their pond is drying out quickly, they can accelerate their transition to land, dropping their gills and heading out of the water. But this pond has remained wet into late spring, and so it’s better to stay there and store up more energy.

“He’s just holding out as long as he can,” Brady says, returning the salamander to the water.

Brady, a graduate student at the Yale School of Forestry & Environmental Studies, has mucked his way through dozens of ponds around Connecticut much like this one, and he’s caught thousands of salamanders and frogs. Along the way, he’s made an important discovery: the animals he studies are evolving. And we humans are driving their evolution.

The health of ponds varies enormously, depending on how close by humans are. Ponds that are close to roads, for example, pick up the pollutants that get washed off the blacktop by rain. It turns out that the amphibians that live in roadside ponds are genetically distinct from animals in woodland ponds, having adapted to their new habitats.

Not long ago, evolutionary biologists were fairly sure that evolution was such a slow process that they couldn’t witness it in their own lifetime. Ecologists who studied the changes in ecosystems didn’t worry about tracking the evolution of the species they observed. But it’s now becoming clear, thanks to studies like Brady’s, that evolution happens fast--and that we humans are among the best engines of evolution on the planet.

The first evidence of evolution in our own time came from exotic places, like the Galapagos Islands, lying hundreds of miles off the coast of Ecuador in the Pacific Ocean. Biologists working on the islands documented changes to the beaks of Darwin’s Finches. When droughts hit the islands, many populations of birds could only survive on hard seeds. The birds with the strong, thick beaks were more likely to escape starvation and reproduce. The birds of the next generation had thicker beaks on average than their parents.

Scientists have found evolution at work in many other species since then. And in a growing number of cases, humans are the force driving their evolution. Humans have become the ultimate top predator, for example, catching over 140 million tons of fish each year. Many fisheries require that small fish get thrown back. As a result, many species of fish have evolved so that they are smaller when they reach adulthood.

More recently, scientists have documented other examples of natural selection not on remote islands or in the deep oceans. Even in a heavily settled place like Connecticut, evolution is at work.

A century ago, much of Connecticut was cleared for farming. Since then, maples and other trees have spread across a great deal of the land. Some ponds that once basked in open sunlight are now cloaked in shade. A decade ago, Yale ecologist David Skelly wondered if the wood frogs living the ponds were adapting to the changing levels of sunlight.

Skelly found ponds in Yale Myers Forest that were cast in dark shade, bathed in bright light, or fell somewhere in between. He studied the frogs in each pond and discovered that the tadpoles in the shaded ponds took up to twice as long to develop as the well-lit ones. He brought eggs from the ponds to his lab at Yale, where he raised them in identical conditions. The eggs from shaded ponds still took longer. Skelly knew that the environment can cause genetically identical organisms to grow differently. A plant that grows in the shade, for example, will grow broad leaves and shallow roots; the same plant put in the sun may grow smaller leaves and deep roots. To test this possibility, Skelly brought frog eggs from shaded ponds and sunny ones to his lab and raised them under identical conditions. The eggs from shaded ponds still took longer to develop. That result told Skelly that the difference between the frog populations was genetically encoded.

The growth rate is not the only thing about the frogs that’s evolving. Skelly also found that frogs in shaded ponds prefer warmer water than frogs in open ponds, for example. What makes Skelly’s findings all the more striking is just how little distance he had to go to find big differences in the frogs. A shaded pond and a well-lit one might be separated by just a couple hundred meters and contained differently adapted frogs.

When Brady came to Skelly’s lab in 2008, he wanted to look for other kinds of human activity that might be driving evolution. He soon realized that roads were a good place to look.

Over the past century, the United States has become crisscrossed by a growing web of roads. All told, their blacktop covers an area greater than the entire state of Indiana. Eighty percent of the United States is within a kilometer of a road. And on those roads, traffic has exploded, quadrupling between 1960 and 2000.

The ecological effects of this growing road network are vast. A million vertebrate animals get killed by vehicles in the United States every day. Roads can cut off the migration routes of animals, and they can also slice habitats into smaller fragments.

Roads are also rivers of pollution. In winter, road crews dump tons of salt to melt ice, and much of the salt ends up in the ground alongside the roads, where it can spread for hundreds of meters. “You can taste the salt in these ponds,” says Brady. Along with the salt come heavy metals scraped from brake pads and catalytic converters, organic compounds from gasoline and diesel and a catalog of other pollutants. “There’s a slew of stuff that comes off of roads,” says Brady.

Brady wondered if the pollution coming into the ponds could drive the evolution of their inhabitants. Hiking through the Yale Myers Forest, he selected five ponds near roads to study, along with five others deep inside the woods. He gathered balls of spotted salamander eggs attached to submerged logs. Brady then built cages out of Rubbermaid containers, which he filled with a mix of salamander eggs from the two different kinds of ponds. He submerged the containers into the ponds--some by roads. Then he waited to see how many of the salamanders managed to hatch and reach adulthood.

In the woodland ponds, Brady found, the woodland salamanders and roadside salamanders did equally well, with about 87 percent of the animals surviving. Both kinds of salamanders fared worse in roadside ponds, presumably due to the harsh pollution. But the woodland salamanders fared a lot worse. More roadside salamanders survived in roadside ponds by a margin of 25 percent.

Brady’s study mirrors what other researchers are finding in other species. As humans disturb ecosystems, the animals and plants that live in them often evolve in response within a few years. To understand this rapid change, ecologists and evolutionary biologists are collaborating to create a new discipline, which has been dubbed “eco-evolutionary dynamics.”

When humans drive this rapid evolution, they can have some unexpectedly big effects. In southern Connecticut, for example, human-driven evolution has turned clear lakes cloudy.

Before the arrival of European colonists, fish known as alewives would swim into many of the lakes from the Long Island Sound to lay their eggs. After colonists built dams, alewives became trapped in some of the lakes. Originally, the alewives feasted on water fleas, wiping out the tiny invertebrates before leaving for the ocean. But Yale ecologist David Post and his colleagues found that that once alewives began to stay year-round in the lakes, they shifted to a mixed diet of water fleas and smaller prey. Their mouths evolved to smaller sizes to feed more efficiently on that diet, and they evolved a preference for smaller animals.

The evolution of the alewives drove the evolution of the water fleas. Originally, they coped with the onslaught of the alewives by growing quickly and reproducing at a tremendous clip. To fuel that growth, they scoured the water for algae, leaving the lakes clear. But once alewives became year-round residents of the lakes, the water fleas evolved to grow more slowly. The algae in the lakes now had enough time to get ahead of the water fleas and build up their numbers. As a result, Post argues, clear lakes have turned cloudy.

Humans did not invent this kind of swift evolution, of course. Long before humans were building dams, for example, changing sea levels trapped ocean fish in lakes, where they then quickly adapted to their new habitat. Environmental toxins have menaced animals and plants for millions of years, as well. But between human transformation of the land, our impact on the climate and our release of pollution, people are putting this kind of evolution into overdrive.

Ironically, though, this evolutionary response may buy conservation biologists a bit of breathing room as they try to save the world’s biodiversity. Thanks to the rapid evolution of roadside salamanders, for example, they’re doing better in their polluted ponds than they would have if they still had woodland genes.
Brady thinks that if we can understand the scope of this evolution, we can figure out which species are adapting and which can’t respond to our impact. “We can do a kind of triage,” says Brady, focusing our efforts first on the most vulnerable species.

Still, Brady warns, evolution is not a get-out-of-jail-free card for species. “There’s a limit to how much they can adapt,” says Brady. If the environment changes too quickly for evolution to keep pace, a species may become extinct--even as it’s adapting.

Another complicating factor is that evolution can sometimes take species in unexpected directions. Brady experienced this Darwinian surprise for himself when he turned his attention from spotted salamanders to the wood frogs that share the same ponds. The frogs responded in an entirely different way. The roadside frogs did worse than the woodland frogs in both types of ponds. Evolution has made them maladapted, rather than well-adapted.

Brady is trying to figure out what’s going on. One possibility is that evolution favors quantity over quality. Brady has observed that the roadside wood frogs lay more eggs than their woodland counterparts. Even if any individual egg is more likely to die, the sheer number of them may ensure reproductive success.

It’s also possible that Brady will find a solution when he looks at the frogs after they mature into adults. The frogs may be evolving adaptations for a challenge they face as adults. It could be something as simple as getting across the road quickly without getting squashed. The mutations that help them survive an adulthood risk may have nasty side effect while they’re growing up.

Even though the salamanders seem to have a more straightforward evolutionary history, Brady still has a lot of questions about them. He doesn’t know yet how exactly they’re warding off the pollutants in the ponds. It’s possible that they are detoxifying them quickly, or somehow shutting them out before they even get in. To get some clues, Brady is going to be collaborating with other scientists to analyze individual salamander cells. He hopes to see which genes are more active in the roadside salamanders. Those genes may be the ones that are evolving right now.

“My guess,” says Brady, “is that all these things are going on.”

Copyright 2012 Carl Zimmer
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