Conclusion
We disentangled the morphological, molecular, and ecological mechanisms underlying adaptive evolution against a background of high-level gene flow in a seemingly barrier-free environment. We showed that the effect of historical processes during the last glacial maximum, in concert with niche-driven ecological differentiation, lead to the establishment of three distinct clades across a widely distributed marine pipefish. Ultimately, our study shows that even the sea environment maintains a vast potential for adaptive radiation and ecological specialization. In fact, when multiple lines of evidence and temporal scales are taken into consideration, it turns out that ‘marine speciation’ is actually far from being a ‘paradox’.