American Society of Naturalists

A membership society whose goal is to advance and to diffuse knowledge of organic evolution and other broad biological principles so as to enhance the conceptual unification of the biological sciences.

“Eco-evolutionary feedbacks predict the time course of rapid life history evolution”

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David N. Reznick, Ronald D. Bassar, Corey A. Handelsman, Cameron K. Ghalambor, Jeff Arendt, Tim Coulson, Tomos Potter, Emily W. Ruell, Julián Torres-Dowdall, Paul Bentzen, and Joseph Travis (Nov 2019)

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Guppies teach us why evolution happens

Guppies, a perennial pet store favorite, have helped a UC Riverside scientist unlock a key question about evolution. Do animals evolve in response to the risk of being eaten, or to the environment that they create in the absence of predators? Turns out, it’s the latter.

Riverside biology professor David Reznick explained that in the wild, guppies can migrate over waterfalls and rapids to places where most predators can’t follow them. Once they arrive in safer terrain, Reznick’s previous research shows they evolve rapidly, becoming genetically distinct from their ancestors. “We already knew that they evolved quickly, but what we didn’t yet understand was why,” Reznick said. In a paper appearing in The American Naturalist, Reznick and his co-authors explain the reason that the tiny fish evolve so quickly in safer waters.

To answer their questions, the scientists traveled to Trinidad, guppies’ native habitat, and did an experiment. They moved guppies from areas in streams where predators were plentiful to areas where predators were mostly absent. Over the course of four years, they studied how the introduced guppies changed in comparison to ones from where they originated. “If guppies evolve because they aren’t at risk of becoming food for other fish, then evolution should be visible right away,” Reznick said. “However, if in the absence of predators, they become abundant and deplete the environment of food, then there will be a lag in detectable changes.”

Guppies from all four streams were marked so they could be tracked over the course of four years. Specifically, in this paper, the scientists considered results for males, which tend to live about five months. They looked at the fishes’ age and size at maturity, which are key traits affecting population growth.

They also tracked how the environment changed as the guppy populations expanded, focusing on the abundance of food such as algae and insects, as well as the presence of other non-predator fish. The finished product includes evidence from the four experimental populations in nature and from laboratory common garden studies of the grandchildren of wild caught fish, mathematical modeling, and quantitative genetic analyses of the one experimental population for which they had a pedigree.

They found a two-to-three-year lag between when guppies were introduced and when males evolved, suggesting the second hypothesis was correct; guppies were first changing their new environments, and then as a result, they turned out to be changing themselves. “The speed of evolution makes it possible to study how it happens. The new news is that organisms can shape their own evolution by changing their environment,” Reznick said.

One of Reznick’s current projects includes applying these concepts to questions about human evolution. “Unlike guppies and other organisms, human population density seems to increase without apparent limit, which increases our impact on our environment and on ourselves,” he said.

Co-authors on this study included Ron Basser, a former PhD student at UC Riverside now assistant professor at Williams College, Joe Travis at Florida State University and Corey Handelsman, Cameron Ghalambor, Emily Ruell, and Julian Torres-Dowdall from Colorado State University, Tim Coulson and Tomos Potter of Oxford University, and Paul Bentzen of Dalhousie University.


Organisms can change their environment and, in so doing, change the selection they experience and how they evolve. Population density is one potential mediator of such interactions because high population densities can impact the ecosystem and reduce resource availability. At present, such interactions are best known from theory and laboratory experiments. Here we quantify the importance of such interactions in nature by transplanting guppies from a stream where they co-occur with predators into tributaries that previously lacked both guppies and predators. If guppies evolve solely because of the immediate reduction in mortality rate, the strength of selection and rate of evolution should be greatest at the outset then decline as the population adapts to its new environment. If indirect effects caused by the increase in guppy population density in the absence of predation prevail, then there should be a lag in guppy evolution because time is required for them to modify their environment. The duration of this lag is predicted to be associated with the environmental modification caused by guppies. We observed a lag in life history evolution associated with increases in population density and altered ecology. How guppies evolved matched predictions derived from evolutionary theory that incorporates such density effects.