Competing Fruit Flies Show Evolution in Action
An experiment by graduate student Daniel Bolnick has captured evolution in action, provided support for a long-standing hypothesis in evolutionary biology, and could help explain how some new species arise from old ones.
Using the fruit fly Drosophila melanogaster, Bolnick has shown that intense competition between members of the same species can drive some individuals into using different habitat niches. Biologists think this separation into niches marks the beginning of the process of forming a new species.
"The process of speciation is a long-standing problem in evolutionary ecology," said Bolnick.
"Dan's work represents a current spate of interest in looking for more empirical evidence," said biologist Peter Wainwright, who is Bolnick's thesis supervisor.
The study was published in the March 22 issue of Nature.
When Charles Darwin arrived in the Galapagos Islands, he noticed that there were many more species of finch in the small islands than on the mainland of South America, said Bolnick. Biologists see the same result in other islands and remote places where a single species has arrived and found itself with few competitors.
Animals and plants have to compete for resources both with other species, and with individuals of their own species. These competitive forces balance each other, said Bolnick. In a big environment such as a continent, competition with other species is more important. But when a species-for example, a finch-enters an environment such as a remote island with few other species already present, competition within the species becomes more important.
Recent theoretical work shows that intense competition in the "middle" of a population drives more variation at the edges, said evolutionary biologist Sergey Nuzhdin. Bolnick's work provides experimental evidence for this, he said.
Bolnick set out to test this hypothesis by studying the effect of competition on Drosophila fruitflies. He set up cages of flies with both normal fly food and food tainted with different amounts of cadmium. Some cages had less food available than others, increasing competition.
At the beginning of the experiment, none of the flies would eat food with any amount of cadmium. However, in cages with high competition, some flies began to switch to the toxic food, and passed this characteristic to their offspring.
Within as few as four generations, groups of flies appeared that were exploiting the different "niches" of the food supply by being able to tolerate different levels of cadmium.
The rate of evolution into niches was fastest in the cages with the highest competition, said Bolnick.
"The response was much faster than had been predicted," he said.
Bolnick did not observe the formation of a new species in the experiment, but biologists think that separation into niches that tend not to interbreed marks the beginning of the process.
Currently biologists have two basic models for how new species form. In one model, a group of individuals is physically isolated from the rest of the species, for example by a mountain range. In the second model, the two groups continue to live side by side, but one group adapts to a different "niche" within the habitat.
These ideas on competition and species formation were first put forward in the 1970s, but new theories developed in the last five years have made it possible to test them, said Wainwright.
Pesticide, Urbanization Linked to Frog Decline
Habitat destruction has caused a decrease in the population of California red-legged frogs (Rana aurora draytonii) and the species therefore recently received federal protection. However, researchers from UC Davis and California State University, Sacramento (CSUS), have found new evidence that their decline may also be pesticide-related.
"It's the first time scientists have been able to link a known declining frog species with pesticides," said Carlos Davidson, an environmental studies professor at CSUS. "We found a very strong association between declines of red-legged frogs and the amount of agricultural land use upwind from the site. It strongly suggests that wind-borne agrochemicals may be contributing to the decline."
In a study that encompassed almost all of California, Davidson and co-author H. Bradley Shaffer, professor of evolution and ecology, and member of the Center for Population Biology, mapped out the disappearance of red-legged frog populations. Using those geographic patterns, they analyzed possible causes of the declines.
Mark Jennings of the U.S. Geological Survey also contributed to the study. Their findings are published in the April issue of the journal Ecological Applications.
The red-legged frog has disappeared from over 70 percent of its historic range in California. The U.S. Fish and Wildlife Service added it to the threatened species list in 1996.
To identify historic concentrations of red-legged frogs, Davidson and Shaffer compared museum records of their habitats, dating back to the mid-1800s, with recent survey data. "From the museum specimens, we know where the frogs used to be. Recent survey data tells where they are now," Davidson said.
Of the 237 sites they looked at that once had frog populations, 48 percent no longer do.
The researchers looked at several possible causes of the declines, including global warming, ultraviolet radiation, pesticide use and habitat destruction due to urbanization and agriculture. They concluded that both urbanization and pesticides might be important factors in the declines.
At each site they calculated the predominant wind direction and the amount of agricultural land use upwind. The percentage of upwind land use in agriculture for sites where the red-legged frog has disappeared was 6 1/2 times greater than for sites where they still exist, suggesting that wind-borne agrochemicals may be an important factor in frog declines.
"The results were consistent," Davidson said. "We found that areas with a lot of agricultural land use upwind are more likely to have declines than sites with less upwind agriculture."
"It's an issue that has impact far beyond California," Davidson added. "There have been amphibian declines in many locations around the world, and pesticides are definitely a possibility. In both Central America and Australia, declines have been found close to major agricultural areas.
"If it turns out pesticides are the cause, we'll have to do more than set aside habitats to protect the species. We'll have to do something about the types and amounts of pesticides that are used and how they are applied."
UC Davis, Santa Barbara Get $6 million To Help Estuaries
University of California researchers at the Davis and Santa Barbara campuses will receive $6 million in federal funds to develop much-needed health assessments for estuaries, the critical coastal ecosystems where fresh water meets salt water.
The grants, from the U.S. Environmental Protection Agency (EPA), will support the development of chemical, biological and physical standards for estuaries, particularly for estuarine wetlands.
Those standards will function rather like the measures of blood pressure, heart function and blood chemistry that are used to assess human health, said Susan Anderson, the project director and an associate research biologist at UC Davis' Bodega Marine Laboratory.
"We need to be able to assess the situation before the patient gets sick. Or if the patient is already sick, to assess how sick. Is it the ecological version of measles,
cancer or just the common cold?" said Roger Nisbet, the project's associate director at UC Santa Barbara and a professor of ecology, evolution and marine biology.
Estuaries are vital components of the planet's ecosystem. They serve as nursery grounds for two-thirds of the fish and shellfish consumed by Americans; provide essential food and habitat for birds, fish and other wildlife; improve water
quality by filtering pollutants; act as buffers to protect shorelines from erosion and flooding; and are the sites of ports and marinas.
But estuaries are in serious trouble. In California, the problems include the loss of more than 90 percent of wetlands; invasive species that threaten natives and their
habitats; and toxic runoff from industry, farms and neighborhoods.
"Public agencies like the EPA and University of California research centers need to assess how our estuaries are doing," Anderson said. "But the health indicators we now have are not sophisticated enough. For instance, when pollution occurs or an exotic species invades, we can't discern subtle effects or predict responses at all levels in the ecosystem.
"Our challenge is to improve those indicators."
The Davis-Santa Barbara center, to be named the Western Center for Estuarine Ecosystem Indicator Research, is one of five regional research centers that will be funded through 2004 by the U.S. EPA. The others are at the Great Lakes, on the Atlantic Coast and on the Gulf of Mexico.
At the western center, about 25 UC scientists will work with each other, local agencies and the other four EPA centers so that their findings will be applicable both locally and nationally.
"The partnership draws on major strengths of both UC Davis and UC Santa Barbara," said Nisbet. "We will create a bridge between state-of-the-art academic ecology and toxicology and practical environmental management."
The center's associate director at UC Davis is Bodega Marine Laboratory professor Gary Cherr. "With our current knowledge, we can easily see whether an estuary is highly impacted or very pristine. The real difficulty comes in assessing estuaries between those two endpoints, where the majority of our West Coast estuaries lie," Cherr said. "The only way to know their status with confidence is to take the very multidisciplinary approach that will be highlighted in the new center."
Laura Yoshii, acting regional administrator for the EPA's Pacific Southwest Office, said, "This funding will go a long way toward helping us learn more about the unique ecosystems found where our rivers, streams and creeks meet the Pacific. We need a better understanding of what impact human activity has had on California's estuaries before we can move forward on developing solutions for these critical habitats. We look forward to working with UC Davis and Santa Barbara to get this much-needed project under way."
|