Figure 4: How facilitation may act as a stepping stone for ecological speciation. We looked at that question by increasing the environmental difference between core and marginal habitats. (a) represents the population divergence at neutral loci with either no benefactor or different patch sizes of benefactor in the marginal environment. (b) represents the divergence at selected loci. (c) represents the average fitness in each environment for these two scenarios. The same model and parameters as in Figure 2 were used, except that we increased environmental difference between core and marginal habitats (Box  1). Without benefactor, no population can establish in the marginal environment that is too harsh for migrants to persist. Only dispersers can be observed there, except in one occasion of actual colonization (outlier at patch size of 0). In presence of benefactors, all environments are occupied by locally adapted populations and gene flow among environments is reduced. Small patch sizes will increase dispersal between the benefactor and marginal environment as in Figure 2.