Pareto fronts reveal constraints on the evolution of ecological niches

Trade-offs are central to biodiversity because they prevent the emergence of dominant phenotypes by limiting the simultaneous optimization of multiple fitness components. Yet trade-offs are often difficult to detect empirically when variation in overall performance produces positive correlations that mask underlying constraints. Here we use Pareto fronts--boundaries that capture optimal trade-off solutions--to test for evolutionary constraints on niche-determining traits in phytoplankton, including minimum nutrient requirements, thermal breadth, salt tolerance, and population growth rates. Using experimentally evolved Chlamydomonas reinhardtii populations subjected to nutrient and salt stress, we detected widespread Pareto fronts limiting the joint optimization of growth rate and niche-determining traits, thereby restricting the emergence of multivariate stress tolerance. Importantly, Pareto fronts revealed trade-offs even when underlying trait correlations were positive. We found that the structure of trait covariation behind Pareto fronts strongly predicted evolutionary outcomes: populations moved toward Pareto-optimal phenotypes primarily when trait correlations were neutral or positive, whereas negative trait correlations were associated with limited evolutionary optimization. Extending this framework across phytoplankton diversity, we compiled niche-determining traits for 189 phytoplankton species. At a macroevolutionary scale, we detected significant Pareto fronts constraining the evolution of niche-determining traits in phytoplankton. These fronts, however, did not always recapitulate the structure of trade-offs evident among C. reinhardtii populations, suggesting that forces that dictate microevolutionary outcomes, such as genetic correlations, can be resolved across macroevolutionary time. Together, our results highlight that evolutionary trajectories may differ across scales, but that fundamental limits on multivariate trait optimization persist across phytoplankton.