One fish’s evolution raises questions about what constitutes a species.
Lindsey Konkel • January 19, 2009
Brilliant blue cichlids like this one produce dull-colored offspring when they mate with red cichlids in
Africa's muddy lake Victoria. Scientists question whether the hybrid is a new species. [Credit: Mean
and Pinchy, flickr.com].
Lake Victoria cichlids are some of the most rapidly evolving animals in the world. In the last 20,000 years, a mere blink of the eye in Earth’s history, more than 500 distinct species of the colorful fish emerged from one common ancestor in the huge east African lake.
What is even more striking is their rapid extinction. In the past 25 years, nearly half of those 500 Lake Victoria cichlid (pronounced sik-lid) species have been lost due to pollution that clouded the lake’s waters. But the cichlid’s loss has illuminated an ongoing controversy over what defines a genuinely distinct species—a question that is at least as murky as Lake Victoria’s waters. What may seem like an arcane dispute among evolutionary biologists is likely to have much wider implications, affecting conservation efforts of many animals and plant species throughout the world.
Lake Victoria cichlids seem to support the idea of some biologists that speciation—the process by which a species evolves into one or more new ones—can occur even when the newly evolved animals and preceding species are genetically indistinguishable. Cloudy lake waters are causing two cichlid species—one mostly red and the other mostly blue—to breed with each other because the fish can no longer see clearly enough to choose mating partners of their own kind. According to biologist Ole Seehausen, of the University of Bern in Switzerland, the offspring, a blandly colored hybrid, represents a new species.
But were those red and blue cichlids ever two truly distinct species? And is the new, dull-colored fish really a new species? Biologists have been wrestling with those and other speciation questions since before Charles Darwin proposed the first scientifically viable theory of how new species come to exist. “No one definition has yet satisfied all naturalists; yet every naturalist knows vaguely what he means when he speaks of a species,” Darwin wrote in The Origin of Species, published in 1859.
Biologists have come a long way since Darwin’s time in understanding the evolutionary forces shaping species, but are still unsure of the best way to broker biodiversity. Some biologists believe a species should be defined solely based on the uniqueness of its DNA; others, including Seehausen argue that a species should be defined by reproductive isolation—the inability or unwillingness of one species to successfully mate with members of other species to produce viable offspring.
Seehausen is convinced that the new, drab-colored fish they have observed in the cloudy waters of Lake Victoria is a legitimately new species. The new hybrid is taking over the habitat of both parent species, but is fulfilling the ecological role of neither, as it retains only some of the unique qualities of each parent species. For example, the new hybrid species is a generalist feeder and specializes on neither plankton nor bottom dwelling insects like its parent species. This concerns Seehausen, who stresses the importance of biodiversity to maintaining a healthy ecosystem.
To Karen Carlton, an evolutionary biologist at the University of Maryland in College Park who studies cichlids with Seehausen, the best way to define a species in this case is to ask the fish themselves. The females seem to know their own species and will usually choose to mate with males of their same color over males with different color patterns.
“There are behavioral cues that make them species in the first place and then keep them separate groups of isolated individuals,” says Carlton.
Many species that arise through reproductive isolation are separated from other populations for long periods of time by some geographic barrier, like a mountain or a river. This seclusion allows different groups of the same species to evolve independently of one another and eventually form separate species. But a geographic barrier may not always be necessary for new species to arise. Seehausen and Carlton contend that an “intrinsic behavioral barrier,” such as female mate preference, is enough to drive speciation in Lake Victoria cichlids.
Other biologists, however, assert that what Seehausen and Carlton are seeing is not true speciation. Genetically, red and blue cichlids are very similar. To Robert Zink, an evolutionary biologist at the University of Minnesota in Minneapolis, this makes them one species. Zink, who studies the evolution of a small bird called the cactus wren, thinks that grouping species based on differences in DNA is a better way to name species and account for biodiversity.
Seehausen counters that this concept does not work for cichlids because they are “too young.” Red and blue cichlids diverged from a common ancestor fewer than 20,000 years ago, which is not sufficient evolutionary time to accumulate enough DNA differences to constitute a new species.
But Zink believes that arguments like Seehausen’s may be hindering rather than helping the cause of biodiversity conservation.
It is simply not practical to save every single subset of a species, or even every distinct species that exists, according to Zink. Yet many biologists have been unwilling to make those tough decisions and have actually worsened the situation by claiming to have identified new species that are not truly distinct, he says.
“Biologists can’t pull the trigger and say, ‘You know what? It’s just not worth saving’,” says Zink. Conservation efforts, he says, should focus first on preserving the most genetically distinct species, even if this means letting others go extinct, so that scarce conservation resources can be reallocated to help the most distinctive forms of life.
But even species that are very similar genetically can make unique and vital contributions to the larger ecosystem, and their disappearance could affect humans in surprising and damaging ways, according to Carlton, the Maryland biologist. For example, some cichlids impact human health by feeding on the freshwater snails that harbor the flatworm that carries schistosomaisis, a debilitating tropical disease. “There can be unexpected ramifications to biodiversity loss,” says Carlton.
The question of how best to define a distinct species is still so scientifically controversial that some researchers question the usefulness of species-based conservation laws. The most famous of those laws, the Endangered Species Act of 1973, which has become the foundation of biodiversity protection in the United States, is flawed, says Zink, because it focuses on species and not on ecosystems. If the focus were on key ecosystems, many species would be protected at the same time.
Other biologists believe a more flexible approach to defining species is the best way to meet conservation goals within the current framework of the Endangered Species Act and similar laws.
“Different species concepts have different strengths and weaknesses,” says Robin Waples, a biologist at the National Oceanic and Atmospheric Administration based in Washington State, who works to conserve Chinook salmon in Puget Sound. He believes that no single concept or definition of species is best under all circumstances. “You can’t really answer which concept makes the most sense without thinking about the goals you are trying to achieve,” he says.
While biologists continue to quarrel over which species definitions work best and what qualities are most worth preserving, time is running out for many groups of organisms. Says Zink, “Our epitaph will read: ‘Conservation biologists fiddled while Rome burned’.”
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The problem of classifying different species certainly needs to be addressed especially since it plays an ever greater role in conservation efforts. Cichlids certainly provides an interesting case study. Great article.