Rapid evolution of genetic and phenotypic divergence in Atlantic salmon following the colonisation of two new branches of a watercourse.

Background: Selection acts strongly on individuals that colonise a habitat and have phenotypic traits that deviate from the local optima. Our objective was to investigate the evolutionary rates in Atlantic salmon (Salmo salar) in a river system (the Vefsna watershed in Norway), fewer than 15 generations after colonisation of two new branches of the watercourse for spawning, which were made available by construction of fish ladders in 1889.

Methods: Differences in age and size were analysed using scale samples collected by anglers.