A reticulate evolutionary history of the three
subspecies
To infer the lineage-splitting pattern within A. marina , we
fitted several models using approximate Bayesian computation (ABC) to
test whether we can distinguish population histories. Our ABC approach
shows that simulated sequences under the model with each subspecies
diverging simultaneously provides the best fit to the observed data
(Figure 5a). This conclusion was validated by three repetitions and high
posterior probability of this model (> 0.6, Table 2). This
result indicates the three subspecies diverged from each other
simultaneously. In contrast, simulations with artificial groups
(Simulation 2) allow no robust model selection.
The BB population morphologically diagnosed as marina shows lower
genetic divergence and differentiation from eucalyptifolia thanmarina (Figure 2). Is it an eucalyptifolia mis-diagnosed
as marina or a marina exchanging genes witheucalyptifolia ? Our ABC simulation (Simulation 3) shows that BB
has descended from marina but experiences gene flow witheucalyptifolia populations (model v2, posterior probability
0.933, Table 2 and Figure 5b). This indicates that subspecies, while
significantly differentiated, are genetically permeable. We also used
TreeMix to capture potential gene flow events among populations. We
identified five such events on the population splitting graph (Table
S7). Three such events occurred between subspecies and the other two
events occurred between BB and the outgroup species A. alba(Figure 5c).