Salinity tolerance drives ecological speciation in amphipods from the northern Chihuahuan Desert

Ecological speciation is the process by which barriers to gene flow evolve between populations as a result of ecologically-based divergent selection.  Environmental salinity has been identified as one of the most important ecological drivers influencing the distribution, abundance, and species richness of aquatic organisms.  Springs of the northern Chihuahuan Desert vary in salinity and are home to the Gammarus pecos (Crustacea: Amphipoda) species complex.  We used a field experiment to compare salinity tolerances among nine amphipod populations from geographically discrete habitats, and to calculate “physiological similarities” among populations.  Cluster analysis placed populations into three groups corresponding to populations from low, medium, and high ambient salinities.  Mantel tests revealed significant correlations between salinity tolerance and habitat salinity, after controlling for other variables such as geographic distance and neutral genetic similarity.  We conclude that ecological speciation has occurred among amphipod populations at different springs, as indicated by dissimilar physiological responses which follow differences in ambient spring salinities.  Reinforced by dispersal barriers between springs, gene flow is restricted between populations and selection preserves variants which most effectively tolerate local salinity levels.  Gammarus diversification in the northern Chihuahuan Desert is driven by vicariance and isolation, along with local selection and adaptation.