Two Secondary Introductions From a Shared Bridgehead Population Show Evidence of Divergent and Parallel Selection

Biological invasions offer large-scale experiments for examining rapid evolution and testing the predictability of adaptive change. Assessing the repeatability of such adaptive responses, however, requires replicated introductions that share similar demographic and environmental contexts. Single introduction events are common during biological invasion, but replicated secondary introductions from a shared source are rare. Here, we take advantage of a unique study system involving the introduced rusa deer (Cervus timorensis) to investigate whether synchronous secondary introductions from a shared bridgehead population show similar evolutionary trajectories. We utilised 5298 DArTSeq Single Nucleotide Polymorphisms (SNPs) from a bridgehead population and two secondary populations to estimate genetic diversity, characterise population structure and detect SNPs putatively under selection. We found an apparent reduction in genetic diversity across serial introductions, consistent with strong founder effects. Population genetic analyses revealed two genetic clusters, indicating strong differentiation and independent evolutionary trajectories between secondary populations. A genome-wide scan using BayPass identified outlier SNPs potentially involved in adaptive responses and some evidence of parallel selection, likely driven by common environmental pressures. We found candidate genes linked to the outlier loci that were associated with functions consistent with adaptive mechanisms. This study highlights that repeated introductions from a common source can further reduce genetic diversity and increase population differentiation, yet still produce parallel adaptive responses under similar environmental conditions.