Divergent evolution of head morphology between marine and freshwater sticklebacks

AbstractIntraspecific phenotypic differentiation is of common place occurrence, but the degree to which it reflects phenotypic plasticity or local adaptation remains often unclear. To be considered as adaptive, the differentiation must be genetically based and exceed what could be expected by neutral processes only. Using laboratory reared full-sib family data from replicate nine-spined stickleback (Pungitius pungitius) populations, we show that freshwater and marine fish display genetically based adaptive differentiation in head size and shape. Utilising identity-by-descent relationships among full-sibs as estimated with the aid of molecular markers, we further show that the studied traits are also highly heritable in all populations indicating and that they can respond to future episodes of natural selection. The head shape and size of pond fish suggests that observed adaptive differentiation has been driven by selection favoring limnetic feeding strategy among the pond fish. Analyses of gill-raker morphology were less conclusive: genetic differentiation was found in gill-raker length (pond > marine) and number, but the degree of divergence in these traits did not exceed neutral expectations. Yet, the direction of divergence in gill raker traits are suggestive of the limnetic feeding mode of pond fish, aligning with the inference from the head morphology analyses.