The spatio-temporal axis for phenotypic change: a comparison of source and translocated Arctic charr populations after 25 generations

Evolution of morphological traits is hypothesized to act on an extended time scale, yet studies have suggested that these changes are possible within a few generations. Trophic polymorphism enabled through niche adaptations and ecological opportunity is one phenomenon that facilitate occurrence of rapid adaptive variation, common in many northern freshwater fish species. One such species is Arctic charr, which is known for its extensive variation in morphology and the occurrence of morphs. However, the speed at which such morphological variation arises is poorly studied despite the importance for understanding the onset of evolution. The aim of this study was to elucidate this process in a gradient of eight lakes that was stocked with Arctic charr in the period from 1910 to 1917 from Lake Tinnsjoen, Norway. We used morphological measurements to test for differences in traits between populations and Haldane and Darwins evolutionary rates to estimate divergence rates in traits. We also tested for correlation between putative genetic and morphological divergence. In addition, we contrasted the morphological divergence with that expected under neutral genetic expectations, using 12 microsatellite markers, to analyze whether and which morphological differences that is following early genetic divergence. A significant genetic differentiation was found between the source population and five of the translocated populations with corresponding differences in morphological traits for four of the populations. Population genetic structuring indicated six different genetic clusters. The translocated populations also exhibited trait divergence estimated with both Haldane and Darwins rates. Differences in morphological traits showed a significant correlation with genetic divergence, and the morphological differences were most likely affected by differences in lake parameters such as maximum depth, lake size and fish community. We conclude that intraspecific morphological and genetic divergence can form on short evolutionary time scales with important implications for conservation and management practices.