Without any enemies to affect their development, wild guppies evolve into a larger, slower-maturing breed, according to a recent study co-authored in part by University researchers recently published in Science magazine.
The guppies in the study evolved at a rate 1,000 times faster than the rate inferred from the records left by guppy fossils.
“These results are particularly poignant for us because, at the outset, many individuals in the scientific community suggested that we would not live long enough to see them,” said David Reznick, a biology professor at the University of California–Riverside and principal author of the study.
The researchers examined guppy evolution in a Riverside, Calif., lab, as well as in a natural population on the Caribbean island of Trinidad.
Throughout the 18-year study, Reznick often traveled to Trinidad to move guppies into different environments.
“(Trinidad) is beautiful,” said Reznick. “It’s your classic mountain streams with crystal clear water and beautiful vegetation hanging over the stream sides. I grew up living through New York winters, and one of my goals — other than doing science — was to see the tropics.”
Reznick studied guppies in two streams, both of which have waterfalls. He moved guppies from their natural habitat in ponds below the falls to ponds above the falls.
Free from large predators, the guppies were able to take more time and put less energy into reproduction. They matured more slowly and had fewer offspring with each generation.
While Reznick researched in Trinidad, the husband-and-wife team of Frank and Ruth Shaw analyzed and interpreted the study’s data at the University.
“We don’t make inferences about the basis of a change in size at maturity or the age of maturity simply by looking at the guppies in their ponds in Trinidad,” said Ruth Shaw, an associate professor of ecology. “This relies on experiments in which guppies are brought back to the lab where they are introduced into common conditions.”
The lab experiments allowed researchers to determine that a genetic change was responsible for the observed changes in the guppy populations.
The Trinidad work was required to show that these genetic changes were the result of natural selection. In the ponds below the falls, predators preferred to eat larger guppies, making smaller guppies more likely to survive.
“The preferential consumption (of larger guppies) by the different predators has been documented in field conditions,” said Ruth Shaw.
The study was funded by a $65,000 grant from the National Science Foundation.
The study’s results support a version of the theory of evolution called “punctuated equilibria.” This view, developed from the basis of fossil record examination, claims that evolution normally proceeds in relatively brief bursts. This contrasts with the traditional Darwinian view that evolution is a slow and steady process.
Although punctuated equilibria has already displaced gradualism as the most widely accepted view, some scientists have questioned whether natural selection alone could produce such rapid change.
“Some paleontologists have suggested that these rapid bursts of change require explanation beyond the standard evolutionary processes that most of us recognize,” said Ruth Shaw. “Our thinking about this study is that it does demonstrate that conventional genetic processes can account for very rapid evolutionary change.”