Moreover, reinforcement can cause reproductive character displacement, i.e. assortative mating leading to increased prezygotic isolation (Harrison 1993). Lower fitness of hybrids favours the evolution of reinforcement, i.e. Prezygotic isolation can be caused by sexual selection and reproductive incompatibility, while post-zygotic isolation may arise due to reduced viability or fitness of hybrids, i.e. Reproductive isolation may exist due to pre- and post-zygotic isolation. 2012), reproductive isolation (Boughman 2001), or due to recent secondary contact of genetically differentiated populations (Kvist et al. Strong genetic structuring of geographically close populations may exist due to physical and climatic barriers (Allendorf et al. The evolution of different parity modes together with reinforcement may thus promote reproductive isolation and rapid speciation, potentially explaining why only six of the almost 40,000 vertebrates belonging to groups consisting of viviparous and oviparous species exhibit bimodal reproduction. Moreover, hybrids were detected in all types of transects, showing that parity mode alone does not necessarily lead to complete reproductive isolation, which suggests that reinforcement may play an important role. Among types of transects, no significant differences existed in genetic and geographic distances, nor number of detected alleles. Introgression in transects spanning oviparous and viviparous clades was one order of magnitude smaller than transects spanning the same reproductive mode and no statistical differences existed between transects spanning the same reproductive mode and control transects. We measured introgression in transects spanning different viviparous clades, different oviparous subclades, transects containing oviparous and viviparous clades, and transects within the same subclade (control transects). Here, we test whether differences in parity mode lead to reproductive isolation by investigating introgression in Zootoca vivipara, a lizard species exhibiting oviparous and viviparous reproduction. Reproductive isolation among differentiated clades may exist due to genetic incompatibilities, sexual selection, differences in parity mode, reduced post-zygotic survival or reproductive success of hybrids. Mechanisms leading to sympatric speciation are diverse and may build up reproductive isolation.
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