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To a large extent, the current distribution and population structure of many species were shaped by the cycles of isolation in glacial refugia and subsequent population expansions. Isolation in, and post-glacial expansion through, heterogeneous environments led to either neutral or adaptive divergence. Norway spruce is no exception, and its current distribution is the consequence of a complex interplay between evolutionary forces. We investigated population differentiation and adaptations of Norway spruce for juvenile growth, diameter of the trunk at breast height and its components: wood density and tracheid traits. Data from 4,461 phenotyped and genotyped Norway spruce from 396 half-sib families in two progeny tests were used to test for divergent selection in the framework of QST vs FST, and to carry out multiple Genome-Wide Association Studies. We show that (i) the macroscopic resultant trait (trunk diameter), unlike its microscopic components (tracheid dimensions) and juvenile growth, was under divergent selection, and (ii) adaptation predated the Last Glaciation Maximum. Altogether, the current variation in these phenotypic traits in Norway spruce is better explained by local adaptation to ancestral environments than to current ones, where populations were partly pre-adapted mainly through growth-related traits.
Competing Interest Statement
The authors have declared no competing interest.
Footnotes
* Addition of QST vs FST, providing evidence of divergent selection for growth-related macroscopic traits.