Cod in the act of evolution
Charles Darwin and his theory of evolution are getting a lot of attention this month, the 200th anniversary of his birth. Much has happened over those two centuries.
Evolution still brings to mind the kind of change that happens over millions of years, such as humans evolving from forerunners of apes. But there is increasing evidence that species can evolve quite quickly, within our lifetimes, and that human intervention in the natural world is speeding up that process.
Take, for example, the cod fish, a New England icon.
Cod, once plentiful in the region's waters, are increasingly rare because of overfishing. But perhaps more important is the fact that large cod are especially rare. The reason: Decades of intense fishing for the largest cod have meant the species has evolved along the lines of the survivors, which is to say, smaller cod.
In a paper on human predation published last month in the Proceedings of the National Academy of Sciences, researchers analyzed data from earlier studies on 29 commercially harvested plants and animals, mostly fish. They found that human harvesting caused changes three times as fast as those observed in natural populations.
In 95 percent of the cases, the result was a decrease in size (or size-related traits, like bighorn sheep horns). And the shrinkage was substantial, an average of 20 percent. In nearly all cases the individuals were also reproducing at much younger ages and smaller sizes.
Taken individually, these observations are not new. University of Maine professor Michael Kinnison, one of the authors of the paper, says the strength of the research is in its breadth, showing consistent and rapid changes in a variety of species from cod to bighorn sheep to ginseng.
Kinnison says some of the observed changes are "plastic" responses. He says the animals have not changed at the genetic level, but are simply modifying their form or behavior in the face of changing conditions but others are also clearly genetic.
A decade ago, Kinnison and a colleague coined the term "contemporary evolution" to describe that which can be seen in a human lifetime. Their concept, he says, is not at odds with the infamous naturalist.
"Darwin had an idea of a continuum of a time scale, he was just focused on a bit longer time scale, and the [statistical] tools weren't there," says Kinnison. "Darwin never actually tried to look for evolution in the real world in his time."
Darwin did recognize that evolution could happen quickly through the kind of extreme artificial selection that animal breeding represents. He was fascinated by pigeon breeding.
The new study of human selection on wild populations augments Darwin's observations, reflecting what happens in the midrange of evolution - somewhere between the contrived breeding of animals for physical qualities (think racehorses) and, at the other end of the spectrum, the purely natural selection that happens in the wild.
You don't need genetics to explain the decline of New England cod, says Teri Frady, an NOAA Fisheries representative. It's the result of decades of overfishing that continues to this day, she says.
Reduction of fishing should allow cod of all sizes to bounce back, as have Georges Bank haddock, she says. But if that fails, she says, another explanation will be needed.
"Until we can have a sustained period of very low fishing rates and see how the stock responds," says Frady, "it's pretty hard to tease out what might be constraining a recovery."
In Canadian waters, cod have not recovered as expected. Douglas Swain, of Fisheries and Oceans Canada, published research in 2007 showing that after a decade of very little fishing, Gulf of St. Lawrence cod remain small for their ages. "This result supports the hypothesis that there have been genetic changes in growth in this population in response to size-selective fishing," according to Swain.
Research on captive fish shows that selective pressure can cause significant genetic changes in just four years. Stony Brook University professor David Conover has found that Atlantic silverside, small fish common along the New England coast, quickly evolve in the opposite direction of selective harvest. Catching the large fish in a population selects for smaller, slower-growing fish; catching the small ones selects for large individuals, and results in greater yields.
Conover said he believes the findings of recent genetic research will eventually be widely applied to fisheries management.
"This is a relatively new idea, and evidence is still building that these evolutionary changes are in fact occurring and therefore we should respond to them," says Conover, making an analogy to the lag time in responding to early scientific evidence for climate change.
The argument that genetic evolution has nothing to do with changes in the wild fish populations flies in the face of the theory of evolution, says Kinnison. Consistently removing the largest individuals from a species should produce some effect, he says; "Evolutionary change is the expectation in biology given these sorts of pressures."
As for the enduring idea that evolution occurs only at a snail's pace, he says, "It would be nice to change that public perception so that folks really understand that evolution is an everyday process . . . going on around us all the time."
Murray Carpenter can be reached at murrcarpenter@gmail.com. 
