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Predators drive rapid changes in guppy reproduction and body shape

An international team of biologists, led by Wageningen University & Research, has demonstrated that predators have a direct and rapid influence on how prey species adapt and evolve. In a controlled experiment with guppies, researchers observed striking changes in reproduction and body shape within just three generations. The findings have been in the journal Science Advances.

Researcher Alexander Kotrschal and his team exposed guppies—small tropical fish—to natural predators in a simulated environment. In each generation, 180 guppies were placed in a tank with a predatory pike cichlid (Crenicichla alta), while 32 control guppies were safely shielded behind a transparent barrier.

Predators as drivers of evolution

Only the survivors were allowed to pass on their genes. This process was repeated over three generations: in each round, guppies were subjected to predation, only the survivors were allowed to reproduce, and their offspring were subjected to the same experiment.

The results speak volumes. Female guppies began reproducing earlier in life and produced larger and more numerous offspring in their first broods. Males developed shorter tails and reproductive organs (gonopodia), likely enhancing their agility and chances of survival.

"It's fascinating to see how quickly evolution can occur," says Alexander Kotrschal. "Within just three generations, we saw clear adaptations. Predators essentially decide who survives and reproduces—thereby shaping the course of evolution. This confirms that they can indeed be a powerful driving force behind evolutionary change."

Not all expectations were met

However, some classic adaptations—such as bright colors or larger eyes—did not emerge. This is notable, as earlier field studies did report such changes. In this experiment, though, there was no sexual selection: the researchers themselves determined which guppies reproduced. In nature, females often choose the most colorful males—which influences which traits are passed on. Other environmental factors, such as food availability, also play a role.

By excluding those influences, this experiment reveals which changes are caused purely by predation. "For example, we saw no difference in color patterns between guppies exposed to predators and those that were not," Kotrschal explains. "That suggests these traits may be more closely linked to other factors, such as sexual selection or food supply."

Evolution under the microscope

This study highlights the value of testing how predators influence the evolution of a species in a controlled setting. Thanks to the smart design of the experiment, the researchers were able—for the first time—to clearly identify which changes are directly caused by predation. Once again, the guppy proves to be an ideal model organism for studying rapid evolutionary adaptation up close.

"What we could previously only infer from field observations, we've now been able to test directly. And that has led to some surprising insights," Kotrschal concludes.