Fig. 3.

a Simulation output obtained with a bi-locus deterministic model with selection against recombinant genotypes (s = 0.1) and a slight advantage of the Mediterranean background (a = 0.01) in a 60-deme Y-shaped stepping-stone model (m = 0.3), including two weak barriers to gene flow (mbar = 0.1) between demes 20 and 21 (AOF) in the northern chain, and between demes 40 and 41 (GBB) in the southern chain of demes, a contact between demes 50 and 51, and unidirectional migration right to the AOF. Above: northern chain, below: southern chain. The inset shows the output when migration is bidirectional between the two chains. Clines are represented for every 500 generations with a rainbow colour code from orange to dark blue. Blue clines superimposed green clines in the southern chain because they remain at the same position (trapping). b Simulation with a contact between demes 10 and 11 and an advantage of the Atlantic background (a = 0.01). Clines are represented for every 500 generations with a rainbow colour code from orange to dark blue. Dark blue clines superimpose light blue and green clines because they remain at the same position (trapping)