Figure 2: Multidimensional scaling plot (MDS) showing the genetic structure of scrubtit samples by population based on Prevosti’s genetic distance. (a) shows all sampled individuals across mainland Tasmania and King Island; (b) shows only King Island samples. Numbers in parentheses on axis titles denote the proportion of total variance explained by the first two coordinates.
Pairwise FST estimates between the King Island subpopulations (0.12 - 0.18) were larger than the majority of the pairwise estimates between the mainland subpopulations (0.01 – 0.18) except the Tasman Peninsula (> 0.22, Table S5). Genetic isolation was positively correlated with geographic distance (glm β = 494.7, se = 101.7, p < .001, McFadden’s R2 = 0.36, Figure S5).
Increasing values of k resulted in decreased values of the cross-entropy criterion (Figure S6), with changes in the criterion value suggesting that the best number of ancestral populations for interpretation within the current sampling design ranged from 2 to 4. Admixture plots showed strong differentiation of the King Island subspecies from the Tasmanian scrubtit regardless of the estimated number of ancestral populations (Figures 3 & S7). When k = 3, the east coast subpopulations in the Tasman Peninsula and north east separated with a high degree of confidence from the remainder of the Tasmanian scrubtit population, and when k = 4 the Tasman Peninsula and north east subpopulations separated from each other. Despite being isolated from the mainland by a 4 km wide sea strait, the Bruny Island subpopulation was less isolated from the rest of the Tasmanian scrubtit population than were the north-eastern and Tasman Peninsula subpopulations (Figures 3 & S7, Table S5). When analysed independently, King Island scrubtits showed a high degree of differentiation by subpopulation when k = 3 (Figures 4, S8 – S9).