Background: The origin of species is a central topic in biology aiming at understanding mechanisms, level and rate of diversification. Ecological speciation is an important driver in adaptive radiation during post glacial intralacustrine niche diversification in fishes. The Arctic charr Salvelinus alpinus L. species complex in the Northern hemisphere freshwater systems display huge morphological and life history divergence in lakes with one or several morphs present, thus offering a unique opportunity to address ongoing speciation mechanisms. We studied Arctic charr in Lake Tinnsjoen by fishing in four nominal lake habitats (pelagial, littoral, shallow moderate profundal, and deep profundal habitats) down to 350 meters depth. Research topics addressed were; (1) to illuminate Holarctic phylogeography and lineages colonizing Lake Tinnsjoen, (2) to estimate reproductive isolation of morphs or fish using unbiased methods, and (3) to document ecomorphological and life history trait divergence. Also, we compared Lake Tinnsjoen with four Norwegian outgroup populations of Arctic charr. Results: Four field assigned morphs were identified in Lake Tinnsjoen; the planktivore morph in all habitats except deep profundal, the dwarf morph in shallow moderate profundal, the piscivore morph in shallow moderate profundal (less in littoral and deep profundal), and an undescribed new morph, being the abyssal morph in the deep profundal only. The morphs displayed extensive life history variation based on age and size patterns. A moderate to high concordance was observed between field assigned morphs and four unbiased genetic clusters obtained from microsatellite variation. MtDNA suggested the occurrence of two minor endemic clades in Lake Tinnsjoen likely originating from one widespread colonizing clade in the Holarctic. All morphs were genetically differentiated at microsatellites (Fst: 0.12 to 0.20; with some ongoing gene flow among morphs, and for most mtDNA comparisons (Fst: 0.04 to 0.38). Analyses of Norwegian outgroup lakes implied colonization from a river system below Lake Tinnsjoen. Conclusion: Our findings suggest postglacial adaptive radiation of one colonizing mtDNA lineage with divergent niche specialization along a depth temperature productivity pressure gradient. Concordance between reproductive isolation and the realized habitat of the morphs imply that ecological speciation may be the mechanism of divergence. Particularly novel is the extensive morph diversification with depth into the often unexplored deep water profundal habitat, suggesting we may have systematically underestimated biodiversity present in lakes. Key words: Adaptive radiation, Ecological speciation, Niche specialization, Population divergence, Morphs, Natural selection, Pleistocene iceage, Microsatellites, MtDNA, Salvelinus alpinus

And if you gaze long into an abyss, the abyss gazes also into thee: four morphs of Arctic charr adapting to a depth-gradient in Lake Tinnsjoen