Evolution and Ecology
Completing Darwin's work on evolution by unlocking the mysteries of how natural selection drives the origin of new species.
"The diversity of life is the most astonishing thing to happen in the universe since the Big Bang, and we need to understand it better if we hope to preserve it."
When Charles Darwin stepped ashore on the Galapagos Islands in 1835, he encountered a group of small, brown birds called finches. He didn't think much of the birds at the time, but back in his office in London, as his ideas about evolution were taking shape, Darwin realized that the finches he had collected were all related to each other yet differed in their beak size and overall shape. Writing in his memoir, The Voyage of the Beagle, he speculated on how that could have happened, noting: "One might really fancy that, from an original paucity of birds in this archipelago, one species had been taken and modified for different ends."
Darwin was on to something.
Darwin "was remarkably insightful, but he was never able to test any of his ideas" about natural selection and the origin of species, says Schluter. So, as a PhD student Schluter did just that. In the late 1970s, he followed in Darwin's footsteps to the Galapagos Islands to test how and why different species of finches became modified for different ends.
The process by which one species gives rise to multiple species exploiting different niches is now known as adaptive radiation, as opposed to other more common patterns of evolution in which a single species changes through time without splitting into many forms. In the case of the finches — which earned the nickname "Darwin's finches" — a single species from mainland South America migrated to the rocky Galapagos, where it diversified into 14 species with distinctive beaks specialized for different diets on different islands: seeds, insects, flowers, and even seabird blood.
Much of Schluter's early work focused on what happens when two ecologically similar species compete for resources. A common outcome of this clash is that the two species evolve to be more different and occupy different niches when they co-exist than when they live alone. Schluter demonstrated this phenomenon, known as "character displacement," in two seed-eating species of Darwin's finches: the small ground finch, Geospiza fuliginosa, and the medium ground finch, G. fortis.
Schluter discovered that on islands where only one species was found, the finches' beaks were fairly similar and intermediate in size. But on the island of Santa Cruz where both species overlapped, the smaller ground finches had smaller beaks and the medium ground finches had larger beaks. Character displacement is now recognized as a universal feature of adaptation, and is known to occur in a wide range of organisms including birds, fish, mammals, reptiles, insects, and plants.
Schluter also helped resolve how the 14 different Darwin's finch species are related to each other. He compared aspects of the birds' body shapes and showed that the oldest species are all insect-feeders, whereas the youngest ones dine on seeds. Because the mainland ancestor of all the Galapagos finches was a seed-eater though, Schluter could show that ancient seed-eating birds must have gone extinct, but that the diet re-evolved on the islands where seeds were abundant.
Adaptive radiation is only part of the puzzle of how species are formed. To maintain separate species once organisms begin to diverge, ecologically distinct forms must choose not to mate with one another, or if they do mate, their DNA must be different enough that hybrid offspring are sick or sterile. This "reproductive isolation" is the hallmark of speciation. According to Schluter, how this occurs in nature is one of the deepest questions in biology. And yet, it is one for which he has made considerable inroads.
"When I took [Schluter] on as a graduate student I had no doubt he had exceptional potential," says Schluter's former mentor, Peter Grant, an evolutionary biologist at Princeton University. "[Schluter] is talented in several ways. First, he has a knack of going to the heart of a problem, and bringing considerable quantitative skills to bear on its solution. Second, he is imaginative, infectiously enthusiastic, and an excellent communicator."
Darwin's finches were a good system to study speciation in action. But Schluter wanted an organism that he could manipulate experimentally to test many of his outstanding questions. So when Schluter started up his own lab at UBC, he swapped to studying a group of fish called three-spined sticklebacks. "I started working on the sticklebacks," he says, "so I could start to address these questions in a system in which experiments were possible." It also didn't hurt that the fish lived a bit closer to home — the sticklebacks Schluter studies are found in the lakes and coastal waters of British Columbia.
A single marine type of stickleback occurs in the salty Pacific waters, but in a few freshwater BC lakes, the 5-cm sticklebacks are found in two different forms, known as benthic and limnetic. The benthics are short-bodied, wide-mouthed, poorly armoured fish that feed on insects at the shallow lake margins; the limnetics, on the other hand, are slender-bodied, narrow-mouthed, well-armoured, open lake predators of zooplankton. In the lab, the two forms can mate, but they remain genetically distinct in the wild because their behaviours are so different that they rarely interbreed.
One of the "most glorious features" of sticklebacks for understanding speciation, says Schluter, is that the two forms have independently arisen four times in inland lakes in BC in a period spanning just 10,000 years. "What hits you over the head when you look at these things is just how similar are the events in which two ecologically differentiated species have formed," he says. "In each case, virtually the same series of adaptations is present."
Schluter has taken the different benthic and limnetic forms back to his lab to show experimentally that reproductive isolation also evolves repeatedly. Although he found that benthic and limnetic sticklebacks usually refused to mate together, two benthics or two limnetics that evolved in complete isolation in different lakes were compatible and content breeders. "Repeatedly we see the same mechanism of reproductive isolation evolve in lock step with adaptation to their environment," says Schluter.
How do the fish repeatedly evolve the same adaptations? "Natural selection is the only process that can do that," he says.
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How can the ordinary processes of natural selection and genetic fluctuations through time result in reproductive isolation, such that animals cannot mate or have only sterile offspring? Schluter says: "We know the process of natural selection is involved but not how it accomplishes what it does. What genes do it? Are mating incompatibilities an incidental byproduct of changed body size?"
--Writer: Elie Dolgin