Rapid and coupled phenotypic and genetic divergence in Icelandic Arctic char (Salvelinus alpinus)

Resource polymorphism has been proposed as an important phase of diversific ation and speciation in vertebrates. Studies of fish in young lakes of the Northern Hemisphere indicate variably advanced cases of adaptive trophic di versification. We have previously proposed a scheme describing this variati on in terms of a gradient of resource-based polymorphic traits, emphasizing flexible behaviour in early phases and morphological divergence in more ad vanced phases. Here, we present data on Arctic char (Salvelinus alpinus) in Icelandic lakes exhibiting a variable degree of phenotypic and genotypic s egregation. We show that (i) the morphs are at different levels of phenotyp ic segregation and reproductive isolation and in one case completely reprod uctively isolated, (ii) morphs within lakes appear to be of intralacustrine origin, and (iii) the morphological and genetic divergence of morphs is co rrelated, suggesting a role for trophic adaptation as a driving force in mo rph segregation.