G C A T T A C G G C A T genes Article Adaptive Divergence under Gene Flow along an Environmental Gradient in Two Coexisting Stickleback Species Thijs M. P. Bal 1,* , Alejandro Llanos-Garrido 2 , Anurag Chaturvedi 3,4 , Io Verdonck 4, Bart Hellemans 4 and Joost A. M. Raeymaekers 1 1 Faculty of Biosciences and Aquaculture, Nord University, N-8049 Bodø, Norway;
[email protected] 2 Department of Organismic & Evolutionary Biology, Harvard University, Cambridge, MA 02138, USA;
[email protected] 3 Department of Ecology and Evolution, University of Lausanne, Biophore Building, 1015 Lausanne, Switzerland;
[email protected] 4 Laboratory of Biodiversity and Evolutionary Genomics, KU Leuven, B-3000 Leuven, Belgium;
[email protected] (I.V.);
[email protected] (B.H.) * Correspondence:
[email protected] Abstract: There is a general and solid theoretical framework to explain how the interplay between natural selection and gene flow affects local adaptation. Yet, to what extent coexisting closely related species evolve collectively or show distinctive evolutionary responses remains a fundamental ques- tion. To address this, we studied the population genetic structure and morphological differentiation of sympatric three-spined and nine-spined stickleback. We conducted genotyping-by-sequencing and morphological trait characterisation using 24 individuals of each species from four lowland brackish water (LBW), four lowland freshwater (LFW) and three upland freshwater (UFW) sites in Citation: Bal, T.M.P.; Llanos-Garrido, Belgium and the Netherlands. This combination of sites allowed us to contrast populations from A.; Chaturvedi, A.; Verdonck, I.; isolated but environmentally similar locations (LFW vs.