Fig. 1.

Schematic classification of speciation models. Each speciation model has to explain the evolution of (post- and prezygotic) reproductive isolation and the evolution of ecological differentiation allowing the stable coexistence of the incipient species. (C) In classical models of speciation (orange boxes and arrows), reproductive isolation is initially caused by external events. The two isolated populations evolve separately, thereby gradually diverging from each other. Divergence is associated with the accumulation of genetic or other incompatibilities which result in a fitness reduction in hybrids and, hence, postzygotic isolation (C1). As a consequence, mechanisms preventing hybridisation are selectively favoured once the external cause of reproductive isolation is removed (reinforcement, C2). Accordingly, externally induced prezygotic isolation is replaced by internal prezygotic isolation mechanisms. Only few classical models address the coexistence problem. (E) In ecological models of speciation (green boxes and arrows) disruptive natural selection leads to ecological differentiation (E1), that is, to the evolution of distinct ecotypes that stably coexist in the face of ecological competition (E2). Reproductive isolation can evolve via two routes. Differentiation can be associated with postzygotic isolation, since hybrids may have reduced fitness because of their disfavoured intermediate phenotype. In this case, prezygotic isolation can subsequently evolve via reinforcement (E3). Alternatively, the traits leading to ecological differentiation are ‘magic traits’ in the sense that they directly lead to assortative mating and, hence, prezygotic reproductive isolation (E4). (S) In sexual selection models of speciation (red boxes and arrows) Fisherian runaway selection leads to the divergence of mating preferences (S1) which induce both prezygotic isolation (S2^a; because females differing in preferences will mate with different types of males) and postzygotic isolation (S2^b; because hybrids do not match the preferences of either type of female). At present, only few sexual selection models address the coexistence problem