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Darwin's fishes: the threespine stickleback of the Pacific Northwest
Now, 200 years after Darwin's birth, the stickleback has become an unlikely superstar of evolutionary science — due largely to research rooted in the Pacific Northwest.
Seattle Times science reporter
ThreespinesticklebacksScientific name: Gasterosteus aculeatus
Range: Oceans and coastal rivers and streams in North America, Japan, northern Europe and northern Asia
Life span: 1-3 years
Size: adults range from 2-4 inches
Habits: Males build nests, court females with a zigzag dance, fan the eggs with their tails and guard the young until they disperse
• Named for the spines on their backs, sticklebacks have armor plates but no scales.
• The study of stickleback behavior helped win the 1973 Nobel Prize in Physiology or Medicine.
• Some marine sticklebacks breed in freshwater.
• Unusual species pairs evolved in island lakes in British Columbia: One feeds on the bottom, and the other feeds in the water column.
Seattle Times science reporter
In his voluminous writings, Charles Darwin made only brief mention of a little fish called the stickleback.
The great naturalist was grappling with the question of how animals select mates, and male sticklebacks were a case in point: They construct nests of sand and weeds, attack rivals and dance feverishly to lure females.
But 200 years after Darwin's birth, the stickleback has become an unlikely superstar of evolutionary science — due largely to research rooted in the Pacific Northwest.
Like the finches and tortoises of the Galápagos Islands that sparked Darwin's revolutionary theory, sticklebacks have adapted to myriad habitats in an evolutionary eye-blink. Scientists in Seattle, British Columbia and elsewhere now are using molecular techniques to study those adaptations, and their work is yielding the clearest insights yet into the way natural selection works at the genetic level.
In some cases, they find nature needs to tweak only a few genes to cause major changes in the stickleback body. Other studies show the fish can evolve to cope with new conditions in as little as 10 years. And though sticklebacks are different from people, there may be evolutionary parallels that could help explain human development and disease.
"The stickleback work is probably the best demonstration of natural selection in all its nuances that we have right now," said evolutionary biologist Andrew Hendry, of McGill University in Montreal. "It's fusing the world of genetics and development with the worlds of ecology and evolution better than any other system I know of."
Sticklebacks are so central to the new wave of evolutionary research that they may well eclipse Darwin's finches in importance and earn the honorary title of "Darwin's fishes."
From finches to fish
University of British Columbia biologist Dolph Schluter was one of the first to see the potential in the innocuous-looking fish in his own backyard. But he got his start working with those famous finches.
As a graduate student in the late 1970s, Schluter camped for months at a time on islands so tiny that he could band every bird. Darwin was dazzled by the variation among finches he found in the remote archipelago, which he visited in 1835 aboard HMS Beagle. In "On the Origin of Species by Means of Natural Selection," published more than 20 years later, he laid out his theory that such differences would evolve as nature weeded out individuals whose beaks couldn't crack local seeds or whose plumage failed to impress the opposite sex. As inherited changes accumulated over generations, he said, new species could emerge. But Darwin believed the process took eons.
The studies in which Schluter participated were a landmark, revealing that Galápagos finches actually evolve from year to year as weather and seed types favor larger or smaller beaks.
In other words, it was possible to measure natural selection and evolution in action.
Electrified by the discovery, Schluter wanted to delve into the process through crossbreeding and other experiments that would have been impossible with rare Galápagos species.
But in the Pacific Northwest, threespine sticklebacks are as common as cloudy days.
Plates, spines, color
Schluter's lab looks like a tropical-fish store, but feels like early spring in Seattle. Sticklebacks like it cool. They live in coastal lakes and streams throughout the Northern Hemisphere, including Lake Washington. Most of the 2- to 3-inch fish in Schluter's tanks are silver, but a crimson flush colors the throat of breeding males and their eyes sparkle an iridescent blue.
Freshwater sticklebacks are among the youngest species on Earth, which makes it easier to follow their evolutionary tracks. Their marine ancestors migrated inland when ice-age glaciers melted 10,000 to 15,000 years ago. By comparison, Darwin's finches have been diverging for millions of years.
In their new freshwater homes, sticklebacks blossomed into dozens of varieties, each slightly different from those in the next drainage. But a few consistent changes occurred everywhere the fish adapted.
They shed some of the heavy armor plates that protect against ocean predators but seem to hinder quick escapes in freshwater; they lost belly spines that proved a handicap with insects grabbing at the young fish from below. Sticklebacks that live on lake bottoms are almost always lighter in color than their marine counterparts, probably for camouflage.
Those patterns repeat around the world. That means it's not chance, but natural selection, behind the wheel.
"One of the beautiful things about the stickleback is that the process of colonizing new lakes and streams from the ocean has played out countless thousands of times," said Stanford University developmental biologist David Kingsley, who has led much of the genetic work on the fish. "You've got all these natural experiments replicated over and over again."
To replicate those experiments yet again — but in a controlled way — Schluter built more than 30 ponds on UBC's Vancouver campus, each up to 75 feet square. He jokingly calls the complex his "evolution accelerator."
For several years, he and his students have been crossing and combining varieties of sticklebacks, throwing them in the ponds and keeping careful track as nature — and natural selection — takes its course.
But Schluter wasn't able to peer inside the sticklebacks' DNA until Kingsley and Katie Peichel, of Seattle's Fred Hutchinson Cancer Research Center, offered up their expertise in molecular biology.
In Darwin's day, heredity was a black box. Children today learn about DNA and genes in elementary school. But most genetic experts work with bacteria, fruit flies and other laboratory animals bred under artificial conditions.
So the goals of the stickleback scientists are ambitious: to unravel the evolutionary history of a wild animal by teasing out the key genes that have changed over time. Using Schluter's ponds as the next best thing to nature, they can conduct experiments to measure evolution and natural selection at work on the basic building blocks of inheritance.
"The only way evolution can happen is through genetic changes that can be passed from generation to generation," Peichel said. "We want to figure out what are the actual genes that cause these fish to have spines on their bellies, or not; to have [armor] plates or not have plates."
By crossbreeding sticklebacks and examining their DNA, Kingsley, Peichel and their colleagues have done just that. They also discovered a gene that determines whether sticklebacks will have light or dark skin.
Model for humans
The fact that a few genes can control big changes like the loss of body armor may explain why sticklebacks have been able to adapt to new environments at lightning speeds.
Marine sticklebacks that recolonized an Alaska lake poisoned in the 1980s began to lose their armor plates within 10 years. In Lake Washington, Peichel documented reverse evolution: Body armor became more common after the lake was cleaned up and sticklebacks weren't able to hide in the murk from predatory trout.
The same handful of genes also seems to play the same roles in sticklebacks that evolved independently in opposite corners of the world. "One of the things we've been most delighted about ... is that very often, the very same mechanisms are controlling the evolution of the same traits in other locations," Kingsley said.
That could mean evolution follows some common paths, or may be guided in some cases by a few powerful genes.
Humans share the stickleback gene for skin color. And, like sticklebacks, some of our ancestors evolved lighter skin as they migrated away from humanity's African birthplace. The gene that causes sticklebacks to lose their pelvic spines may be linked to the loss of hind limbs in whales and manatees, and the crippling condition called club foot in humans. People with a mutant form of the stickleback armor gene lack teeth, hair or sweat glands.
"We think the stickleback is a great model for what's going on in humans," Peichel said.
The big question
While much of the stickleback research has peered into the species' past, Schluter and his student Rowan Barrett are using the new genetic insights to kick-start evolution in their experimental ponds.
To mimic the ancestral sticklebacks' leap to freshwater, they seeded the ponds with marine sticklebacks, which still populate oceans in abundance. They let the fish breed naturally, then began measuring the offspring and sampling their DNA. Fish with the gene for reduced body armor grew faster and bigger and reproduced sooner than fish weighted down by heavy plates.
It's clear evidence of natural selection at work, but it doesn't yet meet the standard of evolution-in-action set by Darwin's finches, Schluter said.
To show that, they still have to prove that fish with reduced body armor excel at passing their genes to the next generation.
The experiment is still running.
Schluter has little doubt the sticklebacks will deliver.
"We're now able to ask and answer questions that were almost unthinkable before," he said. "It's a fabulous time to be studying evolution."
Sandi Doughton: 206-464-2491 or email@example.com
Copyright © 2009 The Seattle Times Company
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