show Abstracthide AbstractTo examine the process of adaptation to a novel environment, we sequenced exomes from house mice sampled across two elevational transects in the Andes. We show house mice in Ecuador and Bolivia colonized high elevation independently. The response to selection at high elevation was largely transect-specific, with a small subset of genes under parallel selection. We also found a number of hypoxia-associated genes that exhibited a threshold effect, that is, a large shift in allele frequency at the highest elevations.