| Summary: | Lake Mývatn, Iceland, is a well-studied lake that has several types of habitats and strong temporal dynamics. The threespine stickleback (Gasterosteus aculeatus) population of the lake has been monitored for decades. Within the population is high gene flow, the phenotype is variable, the population fluctuates in size across years, and variation can be seen in diet. Stickleback are known for their diverse prey consumption and their tendency towards both phenotypic and genetic adaptations in response to different habitats and prey. Using stickleback in Lake Mývatn, it is possible to study diet diversity, individual specialization, and phenotypic variation among sites. Clear connection with sites may indicate adaptive plasticity or adaption. Stickleback were caught in Lake Mývatn in June and their head morphology, gill raker structure, gut length, and stomach content analyzed, and individual specialization calculated. The results showed that there was no correlation between head morphology or all traits combined with diet or individual specialization values. However, when gill raker structure was examined independently, a relationship with diet was found, and there was a close-to-significant relationship with individual specialization. The findings suggest that these results can be related to differences in ecological and environmental conditions among sites resulting in responses to natural selection pressures. However, due to extensive gene flow, the observed phenotypic divergence likely represents adaptive plasticity. This thesis provided insight into the biological diversity and individual specialization among sites within Lake Mývatn and establishes a baseline for future research examining the temporal changes in phenotype, diet, and individual specialization of threespine stickleback in this lake. Mývatn er mikið rannsakað vatn, sem hefur margar mismunandi vistgerðir og miklar sveiflur í tíma. Hornsílastofninn (Gasterosteus aculeatus) í vatninu hefur verið rannsakaður í tugi ára. Í stofninum er mikið ...
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