Body-size and growth-rate divergence among populations of threespine stickleback (Gasterosteus aculeatus) in Cook Inlet, Alaska, USA

Abstract

Body size is one of the most perceptible traits of organisms and is an important fitness proxy in evolutionary studies. Oceanic threespine stickleback (Gasterosteus aculeatus L., 1758) have colonized and adapted to numerous freshwater habitats throughout the Holarctic since the most recent glacial retreat, giving us natural “replicates” of both convergent and divergent evolution. I observed considerable body-size variation among 22 threespine stickleback populations within a small region surrounding Cook Inlet, Alaska, USA. Larger bodied populations tended to have bimodal size-frequency distributions, whereas most smaller bodied populations had unimodal distributions. Bimodal distributions suggested the presence of at least two age classes within large-bodied populations. I used a Bayesian approach to infer mean size of presumed age-1 and age-2+ fish from bimodal size-frequency distributions; I found significant differences in size among populations within ages and sexes, suggesting significant divergence in growth rate among populations. I did not find significant correlations between growth rates (age-specific size) and geographic distances among populations, drainage affiliation, or distance to the sea. Thus, historical processes like isolation by distance, gene flow, or recent common ancestry did not explain differences in growth among populations, suggesting a role for local adaptation or phenotypic plasticity in size divergence.