Identifying the molecular changes that lead to ecological specialization during 
speciation is one of the major goals of molecular evolution. One question that 
remains to be thoroughly investigated is whether ecological specialization 
derives strictly from adaptive changes and their associated trade-offs, or from 
conditionally neutral mutations that accumulate under relaxed selection. We used 
whole-genome sequencing, genome annotation and computational analyses to 
identify genes that have rapidly diverged between two incipient species of 
Saccharomyces paradoxus that occupy different climatic regions along a 
south-west to north-east gradient. As candidate loci for ecological 
specialization, we identified genes that show signatures of adaptation and 
accelerated rates of amino acid substitutions, causing asymmetric evolution 
between lineages. This set of genes includes a glycyl-tRNA-synthetase, GRS2, 
which is known to be transcriptionally induced under heat stress in the model 
and sister species S. cerevisiae. Molecular modelling, expression analysis and 
fitness assays suggest that the accelerated evolution of this gene in the 
Northern lineage may be caused by relaxed selection. GRS2 arose during the 
whole-genome duplication (WGD) that occurred 100 million years ago in the yeast 
lineage. While its ohnolog GRS1 has been preserved in all post-WGD species, GRS2 
has frequently been lost and is evolving rapidly, suggesting that the fate of 
this ohnolog is still to be resolved. Our results suggest that the asymmetric 
evolution of GRS2 between the two incipient S. paradoxus species contributes to 
their restricted climatic distributions and thus that ecological specialization 
derives at least partly from relaxed selection rather than a molecular trade-off 
resulting from adaptive evolution.