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Temporal stability of individual feeding specialization may promote speciation.

1. Inter-individual differences in trophic behaviour are considered important in the disruptive selection process for resource specialization and may represent an early phase in the evolution of polymorphism and adaptive radiation. Here, we provide evidence of high stability of individual trophic niches of a fish predator from a 15-year study. 2. Individual resource specialization was investigated by combining data from analyses of stomach contents (recent trophic niche), trophically transmitted parasites (long-term niche) and trophic morphology (niche adaptations) from single specimens of a postglacial fish (Arctic charr) population sampled from contrasting pelagic and littoral habitats. 3. Based on the relationships between morphology, parasites and diet, high inter-individual temporal consistency of narrow niches (zooplanktivorous vs. benthivorous) was evident through the ontogeny of the charr, indicating low degree of switching both in habitat utilization and feeding strategy of individual fish. Co-occurrence of differently specialized behavioural phenotypes was sustained over multiple generations. 4. The stable long-term habitat and feeding specializations may represent an important initial step in an adaptive radiation process, and our findings suggest a case of sympatric speciation into two incipient forms diverging along the littoral-pelagic resource axis.