Abstract
There is a long tradition in using genetically based color polymorphisms in natural populations to study evolutionary processes. Despite growing evidence for continuous phenotypic variation within discrete morphs, we still know little about how this shapes selective dynamics. Here, using 43 years of plumage color data from a Finnish tawny owl population (Strix aluco), we show that gray and brown morphs exhibit substantial intra-morph variation, which has diverged over time. Plumage in the brown morph became increasingly pigmented, while the gray morph showed an abrupt shift toward lighter coloration. By examining both adult and offspring plumage, we identified morph-specific drivers of these trends: in gray owls, reduced pigmentation appears linked to extreme winters that eroded standing genetic variation, likely constraining their evolutionary response. In contrast, brown morph dynamics were shaped by an interaction between plumage coloration, reproductive success, and breeding timing, along with stronger temperature effects during the pre-fledging period. These findings suggest that intra-morph variation determines each morph’s response to selection pressures, potentially influencing their ability to track shifting phenotypic optima. Our work highlights the relevance of phenotypic variation within genetically discrete morphs for evolutionary processes, including how populations respond to environmental change.
Data availability
We have archived all data necessary to reproduce the results and figures in an online data repository: https://doi.org/10.5281/zenodo.1539270371.
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Acknowledgements
This paper is dedicated to the memory of Kari Ahola, who passed away during the writing of the manuscript. Without him, this and many other projects would not have been possible. We greatly thank Teuvo Karstinen and the other members of KBP for the many hours spent conducting fieldwork and data sharing. We also thank Glaucia Del-Rio, Arthur Porto, and Edward Iwimey-Cook for commenting on a draft of the manuscript and advice on animal models. The work was supported by the Swedish Cultural Foundation (grant numbers 168034 and 188919 to P.K.). A.P. was supported by a Margarita Salas fellowship from the University of Seville (MSALAS-2022-22312). M.D.L. was supported by a Marie Sklodowska Curie Individual fellowship awarded by the European Commission (PhenoDim; Grant No. 898932). This is paper number 25 from Kimpari Bird Projects (KBP).
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A.P., M.D.L., and P.K. conceived the study. E.A. and P.K. collected the data in the field. A.P. and M.D.L. analyzed the data. A.P. and M.D.L. wrote the first draft with inputs from P.K. All authors (A.P., M.D.L., E.A., and P.K.) revised and commented on the manuscript and approved the final version.
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Passarotto, A., Lürig, M.D., Aaltonen, E. et al. Morph-specific selection drives phenotypic divergence in color polymorphic tawny owls (Strix aluco) in Northern Europe.
Commun Biol (2025). https://doi.org/10.1038/s42003-025-09365-1
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DOI: https://doi.org/10.1038/s42003-025-09365-1
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