SCIENCE BIOLOGY : Lowly Guppy Helps Scientists Validate Evolutionary Theory
With the help of the humble guppy, California biologists have verified a standard theory on how natural selection molds the reproductive behavior of a species.
The process has been demonstrated innumerable times in laboratory studies. “But when the (experiment) is played out in the real world, with all the vagaries of nature, it might not come out the same way,” said David A. Reznick of UC Riverside.
“There is something special about doing it in the real world,” he added.
Researchers from UC Riverside and UC Santa Barbara moved a school of guppies from one river in Trinidad to another, where they encountered predators different from those in their original home. The researchers reported in Thursday’s edition of the journal Nature that the animals changed their reproductive behavior at the new location to better withstand the predation--exactly as the scientists expected.
When these guppies were then moved to the laboratory, they continued their new reproductive behavior, indicating that the change was caused by evolution.
“It is the first case in which life-history differences observed in nature have been reproduced in the laboratory,” said ecologist Brian Charlesworth of the University of Chicago.
“The really nice thing . . . is that they predicted what would happen and it happened,” said evolutionary biologist Douglas Futuyma of the State University of New York at Stony Brook.
The study is not a unique proof of evolution, he cautioned. “Natural selection is a reality, and we knew that before. . . . But this will end up in textbooks as an example of how to do experiments and validate theories. It’s really quite lovely.”
Evolutionary theory suggests that the life-history of animals--a catch-all term for such variables as the age of first reproduction, the mortality rate at different ages and the reproductive success of individuals over time--is defined by the habits of their predators.
If predators tend to eat mostly adults, a species will begin reproduction at an earlier age and will have larger numbers of progeny in order to ensure the survival of its own genes.
In contrast, if the predator eats mostly the young of a species, then the adults will postpone reproduction until later in life and will produce fewer and larger young, which will have an increased chance of survival.
All one really has to know to understand patterns of reproduction, Futuyma said, is who eats whom and when.
Researchers have documented a large number of such changes in reproductive and other behaviors in wild animals and other species. The development of antibiotic resistance in bacteria and insecticide resistance in plants are prototypical examples of such adaptation. In fact, one of the authors of the new study, John A. Endler of UC Santa Barbara, recently published a book detailing more than 140 such examples.
But in each of these cases, noted Reznick, there is always the nagging suspicion that some unknown or unrecognized factor may influence the changes. Such so-called comparative studies “are always open to the objection that factors other than those imagined in one’s favorite theory are responsible for the correlations observed,” said Charlesworth.
To test the accepted theory of life-history evolution, Endler, Reznick and Heather Bryga of UC Riverside removed guppies from the Aripo River in Trinidad where the natural predators were cichlids, another species of fish that preys primarily on adults. They transplanted the guppies into a tributary of the Aripo where there were previously no guppies and where the primary predators were killifish, which prey primarily on the young.
After 11 years, corresponding to 30 to 60 generations of the guppies, they found that the transplanted guppies began to reproduce about nine days later on average--at 76.1 days of life rather than 67.5. They also produced fewer progeny per brood and the young were slightly larger.
To demonstrate that these changes were caused by inheritance rather than local conditions, the group captured fish at each site and bred them in the Riverside laboratory. They found that the fish continued the reproductive behavior exhibited in the wild, indicating it was genetic in origin and that predation was the most important controlling factor.
They have observed similar results with two other groups of guppies moved to other tributaries of the Aripo, reinforcing their conclusions.
“They’ve really closed the door on the alternatives with this paper,” said evolutionary biologist Ted Case of UC San Diego.
Interestingly, the guppies are also undergoing a variety of other changes in their new environments, according to Endler, including color patterns, body shapes, mate choice and visual displays of fins during mating. Most of these changes had been predicted.
