Relationships between life history traits and latitude can be directly translated to the predicted response of populations to global change. Here, we assessed the relative importance of additive genetic, environmental, and neutral genetic effects on latitudinal variation of thermal reaction norms of growth rate for populations of the damselfly, Ischnura elegans. At the interpopulation level, reaction norms differed primarily in elevation, suggesting that shorter growth seasons in both high latitude and bivoltine populations select for faster growth rates regardless of environmental temperature. Within populations, heritable variation for growth rate, as well as broad sense (family) genetic variance for plasticity suggests that reaction norms are not constrained. However, partitioning of genetic variance into additive (VA) and non-additive effects revealed no significant additive (sire) genetic variance for plasticity, indicating that reaction norms are not free to evolve. Heritability of growth rates were lower in unfavourable conditions (high temperature), likely due to the combined effects of reduced VA and increased environmental and non-additive genetic variance. Quantitative trait divergence (QST) was greater than neutral genetic divergence (FST) in all cases, showing that selection rather than drift is the main contributor to population divergence in growth rate. Additionally, QST estimates increased with experimental temperatures, indicating the presence of genotype by environment interactions.