Why trait gradients across environments differ within species and across communities: Insights from a theoretical model

Sophie Yang , William K Cornwell, Isaac R. Towers, Daniel S. Falster

Trait-environment relationships within plant species are both flatter on average and more variable than community-mean trends, yet the mechanisms driving this variation remain poorly understood. Classic theory attributes this flattening to maladaptive gene flow, but the theory has been underused and its scope, in particular how multiple factors interact to shape trait slopes, remains largely unexplored. Here, we extend it: we model a multi-species community along a one-dimensional environmental gradient to test how four factors shape adaptation within species through time. We found genetic variance accelerated adaptation and steepened trait slopes, while higher gene flow and steeper environmental gradients slowed adaptation and flattened within-species slopes, compared to optimal or across-species responses. Micro-environmental heterogeneity produced flatter slopes without the density penalty imposed by gene flow. The same dynamics governing trait adaptation also govern species abundance distributions; we show abundant-centre patterns are not a general expectation, but a signal of incomplete adaptation. These results connect eco-evolutionary theory to the empirical literature on within-species trait variation, providing a mechanistic basis for interpreting trait slopes and abundant-centre patterns across environmental gradients.