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Figure 1: How facilitation may interfere with ecological speciation and niche conservatism. These 3D plots sketch ongoing disruptive selection and trait divergence of incipient species. The vertical axis represents the relative fitness of genotypes in a trait-ecological space. We assumed that ecologically marginal conditions (E2) also correspond to peripheral areas. Solid arrows are for gene flow including dispersal of propagules, individuals, and sexual reproduction. Dotted lines denote reproductive isolation. A common starting point is immigration of genotypes from the favorable part of the gradient to marginal/peripheral habitats. (a) Diversifying selection without facilitation, and reproductive isolation that eventually arises as a by-product of trait divergence. (b) Maladapted forms persist in marginal habitats because of facilitation and a maintained gene flow among genotypes prevents ecological speciation. (c) Facilitation allows maladapted forms to persist in a changing environment, that is, niche conservatism.