Group-level phenotypes are idiosyncratic yet unpredictable over development in a clonal fish.

Group-level phenotypes in clonal Amazon mollies are initially stable but become highly unpredictable and shuffled during development around 4-5 weeks of age.

• Why it matters: Understanding how group behaviors develop and change is crucial because they can significantly influence individual fitness, yet the origins and stability of these group-level traits remain unclear, especially in genetically identical groups.
• What they did: Researchers established replicate groups of genetically identical Amazon mollies and continuously tracked their behavior from birth to 55 days, controlling for genetic and environmental factors to observe developmental trajectories.
• The result: Early in development, group behaviors follow predictable patterns driven by individual differences, but over time, social feedback causes group phenotypes to become unstable and shuffled, indicating multiple processes shape group variation.