The solution that Kingsley and HHMI associate Katie Peichel came up with was to focus on organisms that had diverged only recently in evolution—so recently that any barriers to mating could be overcome by artificial fertilization.

After a long search, they settled on a little fish called the three-spine stickleback, which boasts a fearsome armor made of bony plates, much like an armadillo's. The three-spine stickleback needs this armor as protection against predators in the ocean, where it normally lives. But it often breeds in freshwater streams and lakes, as close as possible to the North Pole because it likes the cold.

As the glaciers melted at the end of the last ice age, thousands of new streams and lakes were created along the coasts of North America, Europe, and Asia. Many of these were colonized by three-spine sticklebacks, who then encountered new kinds of food and predators in each lake. "In the 10,000 years or so since then, those sticklebacks underwent a dramatic adaptation to whatever local environment was in the lake," Kinglsey says.

It's a very recent event, he emphasizes. "The ancestral population of sticklebacks is still swimming out in the ocean with its three prominent spines on the back, but in the different lakes it changed into all kinds of shapes: giant sticklebacks, pygmy sticklebacks, two-spined sticklebacks, zero-spined sticklebacks, fully armored sticklebacks, partially armored, and no-armored sticklebacks. It's such a dramatic variation in skeletal structure, in fact, that when naturalists first discovered all these fish, they said, 'This looks like 40 different species of fish,' because of how obvious the skeletal changes were."

Later on, Kingsley continues, scientists realized that all these fish were still closely enough related to produce fertile offspring. Some of the different-looking sticklebacks would not breed with each other naturally, but they could be bred together by artificial fertilization. "There is no genetic incompatibility among these fish," he explains. "If you collect sperm and eggs from sticklebacks that look very different from each other and do artificial fertilization, you get completely viable, fertile hybrids. Then you can do a thorough genetic analysis on them."

He adds that if one tried to do artificial insemination with a bat and a horse, for instance, it wouldn't work at all. "They are too far apart from each other," he says. "The divergence time is too great. It isn't 10,000 years ago but about 65 million years ago that mammals diverged—after the meteor hit the Earth. That's an enormous difference.

"But it's still possible to take a formal genetic approach to working on the differences between sticklebacks, even though the skeletal differences are often great. I mean, you're taking what could be 40 armor plates and turning it into zero armor plates; it's as big a difference as many of the shape variations you'd see between different mammals."

Kingsley's team still works mostly on mouse genetics. But Kingsley and Katie Peichel have made several trips to lakes in British Columbia, Washington, and northern California to collect samples of interesting sticklebacks. They raise these fish in tanks built in a special cold room in the lab.

"Here's a pretty example," says Kingsley, pointing to a tank. "See that guy back there with the red tummy and blue sides? That's a male in breeding colors. There has actually been a huge amount of behavioral work done on sticklebacks, particularly by Nikko Tinbergen, the Danish ecologist who won a Nobel Prize for it in 1973.

"Sticklebacks have an elaborate breeding ritual, and Tinbergen worked out a series of visual cues that trigger each step. Red bellies in males, swollen shapes in females, zigzag dances, and trips to nests all serve as triggers for the next step in the overall mating behavior." Because of this interest, which started 40 to 50 years ago, many scientists have studied stickleback ecology, evolution, and behavior, producing a bibliography of more than 2,000 papers. "We are trying to bring a new, molecular-genetic approach to this organism," says Kingsley. "We have already generated more than a thousand genetic markers and constructed the first genomewide linkage maps of the stickleback. Now we are using these maps to study the genetic changes that occurred in the lakes.

"Each lake is its own little independent evolutionary experiment, and many of these traits have evolved multiple times in different lakes. So we'll be able to ask whether there is only one way to make a fish look a particular way. Are the same genes always involved? That's a very important question in evolutionary biology in general."

In the long term, Kingsley would like to know "what kinds of mutations occur, in what genes, to create the new shapes and behaviors that we see in different species.... In the past decade we've learned a lot about the pathways that create skeletal tissue," he says. "In the next decade I think we'll learn how evolution works to design different organisms."

— Maya Pines

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The bright red underside of the male stickleback (left) shows his eagerness to mate as he approaches a female stickleback with three prominent spines and an alluringly swollen shape.

Photo: ©OSF/Animals Animals

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