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Warming-Induced Microevolution in Tawny Owls
Reference
Karell, P., Ahola, K., Karstinen, T., Valkama, J. and Brommer, J.E. 2011. Climate change drives microevolution in a wild bird. Nature Communications: 10.1038/ncomms1213.

Background
The authors introduce the report of their study by noting that "global climate warming changes the environment of most organisms and is expected to lead to a change in selection pressures with micro-evolutionary consequences that allow the adaptation of organisms to the new environment and thereby long-term population persistence," reporting that "microevolution in response to climate change has been demonstrated in invertebrate populations (Umina et al., 2005; Balanya et al., 2006)," but stating that "these studies lack information on the mechanisms and selective factors linking particular genotypes to climate."

What was done
Pushing forward into the latter uncharted territory, Karell et al. explored "the links between climate change and alteration of the selective regime on a highly heritable phenotypic trait, plumage coloration in the tawny owl (Strix aluco), a common bird of prey throughout the temperate regions of Europe." This they did from 1981 to 2008 in a 250-km2 study area in Southern Finland that is dominated by mixed forests, agricultural areas and small freshwater courses, where "plumage coloration was scored using a semi-continuous ordinal scale (range from 4 (grey) to 14 reddish-brown)) on all breeding individuals on each encounter (N = 1116 records of 491 individuals) using a standardized and repeatable scoring that shows coloration is independent of age and sex." In addition, they scored museum skin specimens collected by the public between 1915 and 1980; and they extracted the data on all records of adult ringed and recaptured tawny owls in Finland to which color morph (grey or brown) had been assigned during 1961-2008. Last of all, they say that "measures of temperature and snow depth from the time window that correlated best with annual tawny owl survival were selected as covariates for further modeling."

What was learned
The five Finnish researchers found that "brown individuals had lower survival than grey ones as snow cover became deeper" and that there was a "lower survival for brown individuals compared with grey ones in cold temperatures," which cooler conditions would be expected to promote a deeper snow cover. Thus, they report that as snow depth decreased over time in response to warming, "there was a time trend in survival of the color morphs from 1981-2008, in which survival of the grey morph was fairly stable across years, whereas survival of brown individuals improved dramatically towards the end of the time series." They also report that this phenotypic change was present on a larger nationwide scale, noting that the increase of the brown morph occurred "all over Finland, involving thousands of individuals," such that "the survival propensities of the morphs have equalized in recent, mild winters."

What it means
Karell et al. remark that the phenotypic change they documented "is unlikely to be caused by genetic drift, because drift is only a major force for changing allele frequencies in small populations." Therefore, "given the strong genetic underpinning of tawny owl morphs," they say they consider the observed phenotypic trend in the proportion of the brown morph as "indicative for a shift in gene frequencies." Hence, they conclude that, to the best of their knowledge, the results of their study constitute "the first empirical evidence of climate-driven change in selection on a heritable trait," providing a whole new perspective on the potential abilities of earth's fauna to successfully deal with the challenges of global warming.

References
Balanya, J., Oller, J.M., Huey, R.B., Gilchrist, G.W. and Serra, L. 2006. Global genetic change tracks global climate warming in Drosophila subobscura. Science 313: 1773-1775.

Umina, P.A., Weeks, A.R., Kearney, M.R., McKechnie, S.W. and Hoffmann, A. 2005. A rapid shift in a classical clinal pattern in Drosophila reflecting climate change. Science 308: 691-693.

Reviewed 29 June 2011