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The theory of divergence by trophic polymorphism, an important part of diversification in vertebrates, has recently been extended to encompass the interplay of developmental, ecological and evolutionary processes (Eco-Evo-Devo dynamics). However, this extended theory doesn’t thoroughly explain the evolution of reproductive isolation, which is unfortunate considering the recent advances from the field of speciation. In this thesis, I argue that the Arctic charr morphs of Thingvallavatn are an ideal system to study how reproductive isolation is embedded within the theory of divergence by resource polymorphism, which I present through five papers. First, I focused on two sympatric morphs, the small-benthic (SB) and the planktivorous (PL) charr. Common-garden experiments showed limited evidence for hybridization to affect the structure of trait covariance in both morphs, regarding morphology, developmental timing and feeding behaviour (Paper I), and personality traits (Paper II). However, information on gene expression variability in embryos indicated that hybridization might influence evolvability (Paper III). Multiple reproductive barriers between the two morphs, involving habitat use, assortative mating and hybrid development were also assessed (Paper IV). Finally, Paper V combines field studies and rearing experiments to explore the interplay between habitat choice and offspring development in the large-benthic (LB) charr, which spawns earlier in the season than the other morphs. The results suggested that LB-charr favour temperature conditions that may delay offspring development. Altogether, these findings provide an overview on reproductive isolation among the Arctic charr morphs of Thingvallavatn and constitute a primer to study speciation in an Eco-Evo-Devo context.