In years where these moths reach peak densities, subalpine birch produce defensive secondary-chemical compounds known to be proteinase inhibitors. It is therefore tempting to speculate that the northern-allele for WW1 represents an adaptation to dealing with accumulated secondary chemical compounds in larval autumnal moths. Although our approach does not allow us to identify the mechanisms leading to positive selection for the WW1 northernallele, we have identified environmental conditions that explain the distribution of the WW1 alleles. Our results indicate that allelic variation in the genomic region associated with WW1 is positively correlated with clines in the climate. These results open up an avenue for studies of functional genetics to identify the genes underlying the various adaptations to ecological/climatic conditions. Further, this marker provides a genetic tool to study how climate exerts selection on a genomic region in a bird and hence would make an excellent candidate to predict population changes that result from expected future changes in climate. These results are interesting given the predictions that coldadapted and mountain Gemfibrozil populations of a diversity of taxa are particularly vulnerable to extinction due to the rapid climate warming. In these habitats populations are limited in their ability to disperse upwards to higher altitude areas or northwards and face disproportionately greater risks of extinction in light of significant climate change. Studies of high elevation plants in Europe predict species loss may be as great as 60 percent due to their inability to disperse from these isolated habitats or adapt to warming conditions. In particular, maps of regional plant species vulnerability show almost perfect concordance with the observed northern-allele frequency distribution in Fennoscandian willow warblers. Future work should focus on cold tolerance and food choice GW8510 experiments to contrast mountain with lowland populations in Fennoscandia, to determine the physiological phenotype associated with the geographical and environmental distribution pattern revealed in this study. Experimental elucidation of physiological differences between genotypes will hopefully lead to the discovery of the direct selective mechanisms linked to these apparent adaptations and the genes associated with this AFLP marker.