While adaptive divergence along environmental gradients has repeatedly been demonstrated, the role of postglacial colonization routes in determining phenotypic variation along gradients has received little attention. Here we used a hierarchical QST-FST approach to separate the roles of adaptive and neutral processes in shaping phenotypic variation in moor frog (Rana arvalis) larval life-histories along a 1700 km latitudinal gradient across northern Europe. This species has colonized Scandinavia via two routes with a contact zone in northern Sweden. By using neutral SNP and common garden phenotypic data from 13 populations at two temperatures, we showed that most of the variation along the gradient occurred between the two colonizing lineages. We found little phenotypic divergence within the lineages, however, all phenotypic traits were strongly diverged between the southern and northern colonization routes, with higher growth and development rates and larger body size in the north. The QST estimates between the colonization routes were four times higher than FST, indicating a prominent role for natural selection. QST within the colonization routes did not generally differ from FST, but we found temperature-dependent adaptive divergence close to the contact zone. These results indicate that lineage-specific variation can account for much of the adaptive divergence along a latitudinal gradient.