Icelandic threespine sticklebacks show parallel morphological differences related to mud and lava habitats in four Icelandic lakes. The level of morphological diversification varies among the lakes, ranging from a population with a wide morphological distribution to a population with clear resource morphs, where morphological diversification was represented in diet differences. These differences in morphological divergence represent differences in the ecological surroundings each population experiences. Icelandic sticklebacks may diverge quickly from their marine ancestors. After as little as 13 years isolation in freshwater, a population of sticklebacks differed from their ancestors and showed segregation towards mud or lava habitats in a newly-created lagoon. These diversifications appear to represent resource polymorphisms, which are common in northern fishes, and may lead to population differentation and speciation. Trophically related sexual dimorphism was common in the sticklebacks. Sexual dimorphism has seldom been reported for the species, but might be result of sexual selection or habitat segregation by the sexes.