Life-history traits do not always evolve in response to changing thermal environments

Patrice Pottier , Liam R Dougherty, Ramakrishnan Vasudeva, David Berger, Steven A Ramm, Berta Canal Domenech, Benjamin Cole, Szymon Marian Drobniak , Marco Graziano, Stefan Lüpold, Abhishek Meena, Natalie Pilakouta, Mads Fristrup Schou, Pedro Simões, Erik I Svensson, Cristina Tuni, Shinichi Nakagawa , Claudia Fricke

Quantifying animals' capacity to evolve in response to changing environments is central to predicting ecological responses to climate change. However, empirical evidence for evolutionary responses to changing temperatures remains mixed, and we know little about how selection alters trait variance or generates trade-offs among traits. Here, we conducted a systematic review and meta-analysis of 44 experimental evolution studies to quantify how three key life-history traits—body size, fecundity, and survival—respond to multi-generational exposure to warm or cold temperature regimes. Contrary to our predictions, we found only weak evidence for directional evolution of these traits in changing environments, and no evidence for trade-offs. Across traits—particularly for body size—changes in trait means and variance were small, heterogeneous, and largely unexplained by moderators expected to influence the strength or detectability of evolutionary responses, including the magnitude of temperature change, and the number of generations of selection. Although responses to cooler temperatures were more consistent than responses to warming, this pattern was based on limited data and should be interpreted cautiously. Our results suggest that empirical support for directional evolutionary trait responses to changing temperatures is weak, making broad generalisations across species and contexts difficult. Considerable unexplained heterogeneity further indicates that evolutionary responses may be context-dependent, potentially shaped by the genetic background of source populations or uncaptured methodological variation. Our synthesis adds to growing evidence that many animal populations have limited capacity for rapid evolutionary adaptation and must instead rely predominantly on phenotypic plasticity to buffer the impacts of climate change.