Table S7.

Results from ABC-RF model selection, ran under eight different combinations of demographic parameters

Ancestral Ne	Bottleneck	Introgression	Choice 1	Posterior probability ± SD	Prior error ± SD
Expansion	Bottleneck	YES	10/10	0.868 ± 0.012	0.357 ± 0.0006
Expansion	Bottleneck	NO	10/10	0.817 ± 0.013	0.233 ± 0.0007
Expansion	Constant Ne	YES	10/10	0.889 ± 0.015	0.386 ± 0.0011
Expansion	Constant Ne	NO	10/10	0.797 ± 0.018	0.275 ± 0.0008
Constant Ne	Bottleneck	YES	10/10	0.939 ± 0.012	0.356 ± 0.0005
Constant Ne	Bottleneck	NO	10/10	0.585 ± 0.024	0.252 ± 0.0006
Constant Ne	Constant Ne	YES	10/10	0.927 ± 0.010	0.354 ± 0.0001
Constant Ne	Constant Ne	NO	10/10	0.564 ± 0.025	0.290 ± 0.0010

Constant ancestral N_e vs. demographic expansion in the ancestral population; constant N_e vs. bottlenecks at the time of invasion, and with or without secondary introgression between the two Baltic lineages. Choice 1 represent the number of independent random forest iterations that identified model 1 as the winner.