Like a negative image of farmers breeding progressively larger chickens and cows, human hunters are making their prey become progressively smaller.

Animals hunted by humans are evolving faster than any other animals of their kind in the wild, according to a new study in the Proceedings of the National Academy of Scientists.

By killing big percentages of the largest, sexually mature big-horn sheep or cod, humans can apply a withering selective pressure on huge numbers of animals. But it's not just that overall harvested animal numbers are shrinking — it's that the individuals themselves are shrinking, too, losing an average of 20 percent of their mass over mere decades.

"You've changed the very essence of what they are and what they can do reproductively," said ecologist Chris Darimont of the University of California, Santa Cruz, lead author of the paper.

Evolution doesn't always move slowly. Biologists have found a growing number of wild examples of rapid evolution, such as the increasing resistance of bacteria. Animal breeding programs have also shown that when humans try, they can alter the size of their animals in just a few generations. One turkey breeding program we covered last year increased the average size of a turkey variety 18 pounds, or 85 percent, over the last 40 years or so. Taken together, it is becoming clear that human behavior can exert incredible evolutionary pressure, generating genetic differences over years that might have taken natural selection millennia to produce.

The paper's findings make sense, given what we know about the occasional speediness of evolution, said Steve Palumbi, a Stanford ecologist and author of the book, The Evolution Explosion: How Humans Cause Rapid Evolutionary Change.

"We've known for a long time we've been accelerating the rate of evolution for little things — bacteria, insects, etc. But this paper is about big things — big-horn sheep and fished populations," Palumbi said. "That raises the bar."

Natural experiments, like those conducted by co-author Thomas Reimchen at the University of Victoria, have also shown that predator-prey relationships specifically can drive rapid changes in the hunted.

Reimchen's work focused on the massive variations of a single species of stickleback fish within a set of 200 isolated lakes that formed a mere 10,000 years ago.

"You have the most amazing differences in body form, especially with defensive traits, and it matches really well with what predators are in the particular lake," Darimont said.

By comparing the data from dozens of previous studies on fast evolutionary change that was induced naturally as with the stickleback, by human pollution or invasive species introduction, or by hunting, Darimont's team found that the highly specific nature of hunting drove never-before-seen changes in animal size and shape.

"From a purely evolutionary perspective, we're excited in that we didn't know before that organisms of this type were capable of changing this quickly," Darimont said. "Human predators awoke the latent ability of organisms to change very, very rapidly. In that context, as a purely scientific evolutionary gee-whiz thing, it's pretty cool. But the impacts are huge."

Those impacts are particularly fraught in populations harvested commercially, like many species of fish.

"The fact is that you and I can observe that these salmon are much smaller. And so can the whole food web," Darimont said. "Size is really, really important in nature in determining how species interact. If there is one [species] that has changed dramatically in size, its relationship to natural predators could be lost."

Beyond the size changes, many species are reproducing an average of 25 percent earlier, too.

"As a consequence of targeting large adults, we're targeting reproductive age adults, so those that have genes that say, 'I'm going to reproduce early' have an evolutionary advantage," Darimont explained. "They can have their babies before they enter our rifle scopes or gill nets."

The problem is that these younger mothers don't produce as many babies, which could have major impacts on how humans estimate fishery population and potential. A small change in the age of a fish when it reproduces can change the number of babies it produces by up to an order of magnitude.

"The shift to breeding at younger ages and smaller sizes has a disproportionate effect on how fecund those individuals are," Darimont said.

However, while the work convincingly shows harvested animals that are smaller behave differently, it doesn't explain the mechanisms behind those changes,

Stanford's Palumbi said.

Scientists aren't sure whether the new smaller sizes, for example, reflect modifications in the actual genetic code or whether a more subtle type of adaptation — known as plasticity or simply acclimation — accounts for the differences. Under the second mechanism, the animals would retain their genes but use them differently to adjust to new environmental circumstances. Either mechanism, or likely some combination of the mechanisms, could account for the variation.

A 2003 study on red squirrels in the Yukon

(.pdf) found that actual genetic changes in response to global warming accounted for about 15 percent of the shift in the red squirrel's reproductive behaviors.

The amount of the shift that is actually genetic is important, Palumbi said, because even if you could stop hunters and fishermen from pressuring the population, genetically altered animals would have to re-evolve their previous phenotypes, or forms.

"If it was acclimation, then the individuals who were in the population could just change back. You wouldn't need the whole evolutionary process to happen," he said. "The phenotypic plasticity would be an easier thing to recover from."

Both Darimont and Palumbi agreed, though, that from an ecological management standpoint, whether the animals changed because of their genes or how they used them, the same human behaviors would be required to restore a more natural order to the ecosystem.

"In the context of what we refer to as phenotypic restoration — the idea of restoring old phenotypes — ideally, you could impose moratoria, close fishing or hunting in areas, or you could much more closely mimic natural predators," Darimont said.

That would mean taking younger fish or animals, not the full-grown adults that humans, and the machines that process our food, prefer.

"Most natural predators like to eat the newly born or the almost dead," Darimont said. "Predators focus on babies."

In many states, younger animals are actually protected from hunting and fishing, requiring their release while older animals are fair game.

"A lot of the harvesting regulations say that if you catch a small salmon, you have to put it back because those are future breeders,"

said Darimont. "In the context of evolutionary management, it has a perverse effect."

Darimont said what he called "evolutionarily enlightened" fishing and hunting management was slowly taking hold, as the realization has sunk in that the sheer number of fish in the pond isn't the only important metric for the system.

"Harvest managers often don’t recognize differences among individuals. That's where evolutionary biologists start," Darimont said. "There's an incredible amount of diversity among individuals, and this matters because they might differ in how many babies they have and what they look like, and that sets in motion a whole bunch of things."

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