As global temperatures continue their relentless rise, countless species are on the move, shifting their geographic ranges in search of suitable habitats. Among these, the wall brown butterfly (Lasiommata megera) provides a compelling case study of how rapid evolutionary changes can facilitate range expansions. However, new research published in the prestigious journal Proceedings of the National Academy of Sciences reveals that there are hard limits to how far these insects can travel northwards—limitations shaped not just by warming temperatures but by the unforgiving grip of winter cold.
The wall brown butterfly, a common inhabitant of European grasslands, has been experiencing a notable northward shift across Scandinavia over the past few decades. Interestingly, while the butterfly’s populations have waned in Western Europe, their numbers in northern latitudes have surged. This phenomenon has provoked scientists to investigate the interplay between climate change, evolutionary adaptation, and species distribution. The study, led by an interdisciplinary team at Stockholm University, sought to unravel whether rapid evolution could indeed enable these butterflies to colonize colder regions that were previously inhospitable.
To explore these questions, the researchers undertook a detailed field experiment spanning two years. Butterflies collected from distinct latitudinal regions in Sweden—southern Skåne and more northern counties such as Södermanland and Uppland—were relocated into controlled field cages positioned both within and beyond their current range. By transplanting individuals into southern and northern environments, including a site in southern Dalarna where wall browns have yet to establish, the team could meticulously observe life-history traits under uniform environmental pressures. This experimental design allowed a unique insight into how evolution had shaped the physiological and behavioral strategies of different populations in the face of shifting climates.
One of the critical adaptations the researchers examined was the timing of the butterfly’s life cycle, particularly its alignment with seasonal cues such as daylength and temperature fluctuations. As the wall brown ventured northwards, it faced shorter summers and longer, harsher winters. Remarkably, northern populations exhibited evolutionary changes that allowed them to accelerate their growth and development, effectively compressing their life cycle to fit the constrained growing season. In addition, these butterflies evolved finely tuned mechanisms to initiate winter dormancy at the appropriate time, which is essential for surviving prolonged cold periods.
Yet, despite these impressive evolutionary adjustments, the study uncovered a stark reality: the survival rates of caterpillars during winter varied drastically depending on location. In Dalarna, the territory north of the species’ current range, only a small fraction of the overwintering larvae survived to maturity. This stark reduction in survivorship pointed to cold winter temperatures as a critical ecological barrier. It indicated that evolutionary adaptation to seasonal timing, while beneficial, cannot single-handedly overcome the physiological challenges imposed by extreme cold.
These findings carry profound implications for our understanding of species’ responses to climate change. Often, models predicting range shifts focus on factors such as temperature and precipitation changes, assuming that species can adapt quickly enough to thrive in new environments. However, this research emphasizes that some physiological thresholds—like cold tolerance—set hard boundaries that evolution alone may be unable to breach, at least within short time frames. For the wall brown butterfly, warmer summers and altered daylength responses contribute to expansion, but the persistence of severe winters poses a non-negotiable limit.
A notable aspect of this investigation is its broader relevance beyond a single butterfly species. Many insects and other ectothermic organisms face similar challenges as they attempt to track shifting climate envelopes. Pests, disease vectors, and pollinators alike may encounter comparable evolutionary bottlenecks tied to critical survival traits. Understanding which traits can evolve rapidly and which impose strict constraints is vital for accurate forecasting of ecosystem dynamics, agricultural impacts, and biodiversity conservation under climate change scenarios.
The detailed methodology of this observational study highlights the value of combining experimental translocations with evolutionary ecology. By assessing growth rates, dormancy timing, and overwinter survival under shared environmental conditions, the researchers could disentangle plastic responses from genetically based adaptations. This differentiation is crucial for constructing mechanistic models that predict species’ future distributions with higher fidelity. In doing so, the study exemplifies how evolutionary biology principles can enrich climate impact assessments and biodiversity management strategies.
Among the remarkable discoveries was the demonstration that northern butterfly populations have evolved faster developmental rates to cope with shorter summers. This evolutionary acceleration enables the caterpillars to complete their life cycle before the onset of winter dormancy. Additionally, fine-tuning of seasonal cues shows evolutionary plasticity in responding to photoperiodic signals. However, rapid evolution in these traits did not correlate with enhanced cold tolerance, underscoring the multifaceted nature of adaptive evolution and the distinct physiological constraints imposed by freezing temperatures.
Winter survival proved to be the critical bottleneck. Larvae transplanted beyond the northern boundary existing in Dalarna succumbed in high numbers during the coldest months. This finding suggests the presence of a physiological threshold—in this case, cold hardiness—that evolution has yet to overcome. The temperature extremes and freeze-thaw cycles typical of northern latitudes create a harsh environment that demands specific adaptations such as antifreeze proteins or other cryoprotectants, which may evolve on much longer time scales or require different genetic architectures.
This raises an intriguing question about the capacity for further range expansions in a warming world. While global average temperatures are rising, regional climatic variability, particularly harsh winter episodes, remains a formidable challenge. The study’s authors warn that assuming all species can continuously expand poleward may be overly simplistic and overlook critical evolutionary and ecological limits. These limits, manifest in traits related to overwintering survival, could dictate the maximum extent to which organisms can track optimal climates.
Another key message from this research centers on the interconnectedness of traits and survival strategies. The interplay between life history timing, growth rates, and cold tolerance reveals that evolution does not happen in isolation but as a complex orchestration of multiple adaptations. Successful colonization of new territories may require concurrent evolution in diverse physiological traits, some of which may have inherently slower evolutionary potentials. Consequently, even in the face of rapid climate change, species distributions will continue to be influenced by rigid biological constraints.
The wall brown butterfly case study also provides a cautionary tale for ecological forecasting and conservation planning. Ignoring evolutionary limits could lead to overestimating the resilience of species or underestimating the persistence of cold refugia. The findings highlight the critical need for integrative approaches that incorporate evolutionary biology, ecology, and climatology to anticipate how species and ecosystems will respond to the dual pressures of global warming and climatic extremes.
In conclusion, the study presents compelling evidence that rapid evolution can indeed facilitate range expansions by allowing species to adjust key life history traits. Yet, it simultaneously makes clear that this evolutionary potential has bounds defined by the harsh realities of environmental stressors such as winter cold. For the wall brown butterfly, milder winters may eventually enable further northward spread, but until then, evolution alone cannot surmount the challenge posed by freezing temperatures. This nuanced understanding enriches our broader knowledge of climate-driven biogeographic shifts and provides critical insights for predicting biodiversity patterns in an era of unprecedented environmental change.
Subject of Research: Animals
Article Title: Winters restrict a climate change-driven butterfly range expansion despite rapid evolution of seasonal timing traits
News Publication Date: 23-Jun-2025
Web References:
https://www.pnas.org/cgi/doi/10.1073/pnas.2418392122
References:
DOI: 10.1073/pnas.2418392122
Image Credits: Mats Ittonen
Keywords:
Climate change, butterfly range expansion, wall brown butterfly, Lasiommata megera, rapid evolution, seasonal timing traits, cold tolerance, overwinter survival, ecological limits, latitudinal shifts, evolutionary ecology, species distribution.
