Scientists followed seed beetles for nearly 40 years (about 200 generations) to test how sexual selection affects evolution. Beetles allowed to mate under strong sexual selection diverged genetically and in reproductive traits faster than beetles in enforced monogamy. Researchers observed changes in male seminal proteins and corresponding shifts in female receptors, suggesting coevolution driven by mating competition. The authors note natural selection cannot be fully excluded and call for further molecular and experimental tests.
40 Years of Beetle Mating Shows Sexual Selection Can Speed Evolution
Scientists Saw Evolution Speed Up in BeetlesSergey Spritnyuk - Getty Images
Researchers tracked seed beetles for nearly four decades to test how mating competition shapes evolution. In a long-running lab experiment led by teams at Uppsala University (Sweden) and the University of Belgrade (Serbia), populations of the seed beetle Acanthoscelides obtectus were propagated for roughly 200 generations under two mating regimes: one with free male competition (strong sexual selection) and one with enforced monogamy (weak sexual selection).
What the Experiment Found
Because these beetles have short lifecycles and adapt readily to lab conditions, the researchers could observe genetic and reproductive changes unfold in real time. Populations evolving under strong sexual selection diverged both genetically and in reproductive traits faster and in different directions than populations under weak sexual selection. The researchers report that divergence proceeded more than twice as rapidly where male competition was strong.
“Previous research has suggested that competition between males plays an important role in the formation of new species,” said Göran Arnqvist, lead author from Uppsala. “The populations that evolved under conditions of strong sexual selection ended up being far more different from each other—both genetically and in reproductive characteristics.”
Reproductive Changes Observed
Males from the strong-selection lines evolved altered blends of seminal peptides and proteins compared with males from the weak-selection lines. These ejaculate components — sometimes described as a "copulatory cocktail" — can influence female physiology and post-mating outcomes. Corresponding changes were also detected in female receptors, suggesting coevolution between male ejaculate traits and female biology even though both treatments experienced identical environmental conditions.
Limits and Future Directions
The study, published in Evolution Letters, presents strong evidence that sexual selection can accelerate evolutionary divergence. The authors caution, however, that natural selection may still have contributed to the differences and that the causal link between sexual selection and specific protein changes remains inferred rather than fully demonstrated. They recommend follow-up experiments and molecular tests to more directly connect sexual selection strength to genomic and proteomic evolution.
Arnqvist added that while 200 generations reveal clear directional change, speciation typically requires much longer timescales: over tens or hundreds of thousands of generations, such divergence could potentially lead to reproductive isolation.
Why It Matters: The experiment provides rare, long-term experimental evidence that sexual selection — competition over mates — can be a powerful driver of rapid evolutionary change and may play a key role in the early stages of speciation.
