Modeling local adaptation and gene flow in sockeye salmon

Abstract

Microevolutionary processes determine levels of local adaptation within populations and presumably affect population productivity, but selection and phenotypic evolution have not often been linked explicitly to population dynamics in the wild. Here, we describe a stochastic, individual-based model that simulates evolutionary and demographic effects of migration and selection in interconnected sockeye salmon populations. Two populations were simulated based on parameters obtained empirically from wild populations in the Bristol Bay region of southwestern Alaska, representing beach- A nd stream-spawning ecotypes. Individuals underwent a full salmonid life cycle, experiencing sexual selection, size-selective harvest, and predation based on body size at maturity. Stabilizing natural selection on the three traits (body length, body depth, and age at maturity) tracked for all individuals favored different phenotypes in the two ecotype populations, and the three traits evolved in a genetically correlated manner. Simulation results showed that stabilizing selection on fish phenotypes was always critical for maintaining local adaptation, especially when dispersal rates were high, but loss of local adaptation did not result in substantial loss of productivity. Rather, productivity was more strongly influenced by the opposing effects of stabilizing and harvest selection; strong stabilizing selection caused the salmon to evolve larger body sizes that made them more likely to be caught in the fishery. The model results suggest that interactions between different selection pressures can have substantial demographic as well as evolutionary consequences in wild salmon populations, with key implications for sustainability of natural production in the face of selective harvest and systemic environmental change.