Mechanisms of phenotypic trade-offs in the resource acquisition-allocation Y-model

Abstract Phenotypic trade-offs, predicted to occur due to resource constraints, are not commonly observed. This discrepancy is often explained by the Y-model, which shows that greater variation in resource acquisition (CV A ) than variation in allocation strategy (CV X ) masks among-individual phenotypic trade-offs. However, the Y-model is a heuristic rather than a quantitative tool with testable predictions. Additionally, the mechanisms modulating trade-offs in the model remain unclear. Here, we simulate different parameters of the Y-model to understand their influence. We find that ‘CV A /(CV A + CV X )’ accurately predicts the direction and strength of phenotypic correlations. Contrary to common assumptions, the mean resource acquired by a population does not influence trade-offs; instead, the mean allocation strategy of a population does. Importantly, within-individual dependence of allocation on acquisition exacerbates the influence of mean allocation. This causes greater sensitivity of phenotypic correlations to changes in CV A or CV X , compared to independent resource acquisition and allocation strategies. We generate novel, testable hypotheses about trade-offs in the context of dietary restriction, plasticity, polymorphism, and pace-of-life, and validate our model with empirical data to demonstrate its robustness. By systematically partitioning the influence of means, variances, and covariance, in the acquisition-allocation Y-model, we provide a simple, generalisable, quantitative synthesis for understanding phenotypic trade-offs.