| Summary: | When individuals of a single species encounter different environments, local adaptation and thus phenotypic diversification among populations is expected. This intraspecific diversity, which is common among northern freshwater fish, can be observed even between populations that live in close proximity (sympatry). Common ecological selection pressures often lead to parallel evolution as similar habitats favour the same phenotypic optima resulting in independent evolution of the same traits towards the same direction. Local adaptation is reflected in the external morphology of the fish as many morphological traits are associated with performance and fitness i.e. locomotion activity, foraging efficiency and protection against predators. However, in environments characterized by great niche availability utilization of vacant niches can result in increased intrapopulation phenotypic variation. Differential exploitation and specialization to new alternative resources can also occur between sexes of the same population. This intersexual competition for resources can lead to sexual dimorphism and its extent has been found to increase with niche availability as sexes have the potential to explore a wider range of alternative niches. An important ecological factor that can contribute to divergence among populations is the interaction between hosts and parasites. According to theory, differential parasitism between host populations has the potential to promote adaptation to local environments and thus rapid divergence due to host-parasite co-evolutionary interactions. Parasite mediated selection has been found to operate on the highly polymorphic genes of the major histocompatibility complex (MHC) which is part of the adaptive immune system of the jawed vertebrates. In the first part of the thesis (manuscript I) threespine sticklebacks (Gasterosteus aculeatus) sampled from four lakes located in South-Western Iceland, two of which are inhabited by two distinct stickleback morphs, apparently adapted to different benthic ...
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