The study of wild guppies on the West Indies island of Trinidad, reported in the newest (March 28) issue of the journal Science, found that the fish could evolve between 10,000 and 10 million times faster than the rate of evolution inferred from the fossil record. It suggests that characterizing evolution from the palaeontological record alone may yield a misleading picture.
"We feel that our work is part of a growing body of studies that clearly demonstrates that it is possible to evaluate evolution with experiments in natural populations," David Reznick, professor of biology at the University of California at Riverside and lead author of the study said.
The team's findings add fodder to a current scientific debate. "The perennial problem in evolution is relating things that we can observe in real time to long-term patterns that are unobservable in our lifetime, but are traced out in the fossil record," said Reznick. "The question is whether or not we can explain larger-scale events of evolution in terms of what we can see and study."
In the study of wild guppies in Trinidad, Reznick, along with researchers Frank Shaw and Ruth Shaw of the University of Minnesota, St. Paul, and F. Helen Rodd of the University of California, Davis found that fish that were moved from a predator-infested pool to a pool with just one predator grew larger, lived longer and produced fewer but larger offspring. In the span of seven to 18 generations -- between four and 11 years -- they became more like the native guppies in the relatively predator-free environment.
The NSF-funded study took place in a unique river system in Trinidad where populations of fish are, for the most part, separated from one another by a series of waterfalls. Downstream guppies coexist with at least three predators, including two species of cichlids, the wild version of the common aquarium fish. Guppies in that downstream pool have a generally high mortality rate, become mature at an earlier age, are relatively small at maturity and produce litters more frequently. Upstream guppies, which share a pool with the omnivorous killifish which only sometimes preys on guppies, have a lower mortality, grow larger and have fewer but larger offspring.
The study, he said, shows it is possible to use short-term experiments of natural selection to gain a greater understanding of evolutionary changes that occur over millions of years. "Such studies are important because people tend to think of evolution as a historical process that is not subject to experiments. It is the scarcity of experiments that is the source of some of the criticism of the theory."
Reznick stressed that formation of new species is not addressed by the study, but noted that body size -- one of the principal traits used in palaeontology to distinguish evolutionary changes from formation of new species -- was the prominent characteristic adapted in the guppies studied.
"While the amount of change that we have witnessed seems small, if this rate of change persisted for even 1,000 years, it would result in a radically different animal," Reznick observed.