- The relationship between microevolution and macroevolution is a topic of fundamental importance in evolutionary biology. The increasing accessibility of genomic tools is making the hunt for genes that underlie evolutionary divergence more tractable and, when combined with physiological approaches, provides exceptional power to elucidate the causal mechanisms of the relationship. In this issue of Molecular Ecology, Whitehead et al. (2013) employ this strategy to show that common physiological and genomic mechanisms lead to divergence in salinity tolerance across micro- and macroevolutionary timescales. They compare two killifish species from the genus Fundulus, F. majalis, which inhabits primarily marine and brackish environments and represents the ancestral state of the genus, and F. heteroclitus, which has derived an osmotic niche that expands into freshwater. Corresponding to the differences in osmotic niche, the species differ strikingly in how the structure of the ion-transporting epithelium and the transcriptome of the gills respond to osmotic challenge. These inter-specific differences were similar to but more pronounced than the differences associated with the more subtle intra-specific variation in osmotic niche within each species. It appears that a progression of the same functional adjustments first allowed expansion of the osmotic niche of F. heteroclitus into freshwater and then further expanded the niche of select F. heteroclitus populations towards more dilute freshwater environments. The work of Whitehead et al. therefore emphasizes how the mechanisms of adaptive divergence between populations can be expanded over time to produce the more complex differences that can exist between species.