Non-adaptive evolutionary processes governed the diversification of a temperate conifer lineage after its migration into the tropics

•A phylogeny of Mexican firs (Abies, Pinaceae) was obtained from GBS data.•Modern species derived from a single North American lineage that migrated southwards.•Divergence was driven by non-adaptive allopatric processes and secondary contact.•Lineages have diminishing efficiency of purifying selection with decreasing latitude.