Lucas Marie-Orleach1,2,3,8, Sylvain Glémin2,4,8, Marie K. Brandrud1, Anne K. Brysting5, Abel Gizaw1, A. Lovisa S. Gustafsson1, Loren H. Rieseberg6, Christian Brochmann1,8 and Siri Birkeland1,7,8 1Natural History Museum, University of Oslo, 0562 Oslo, Norway 2CNRS, Université de Rennes, ECOBIO–UMR 6553, Campus de Beaulieu, Rennes 35042, France 3Institut de Recherche sur la Biologie de l'Insecte, UMR 7261, CNRS–Université de Tours, Tours 37200, France 4Department of Ecology and Genetics, Evolutionary Biology Centre, Uppsala University, Evolutionsbiologiskt Centrum EBC, Uppsala, Sweden 5Centre for Ecological and Evolutionary Synthesis, Department of Biosciences, University of Oslo, 0371 Oslo, Norway 6Department of Botany and Biodiversity Research Centre, University of British Columbia, Vancouver, British Columbia, Canada V6T 1Z4 7Department of Chemistry, Biotechnology and Food Science, Norwegian University of Life Sciences, 1433 Ås, Norway Correspondence: lucas.marie-orleachuniv-tours.fr; siri.birkelandnhm.uio.no

↵8 These authors contributed equally to this work.

Surprisingly little attention has been given to the impact of selfing on speciation, even though selfing reduces gene flow between populations and affects other key population genetics parameters. Here we review recent theoretical work and compile empirical data from crossing experiments and genomic and phylogenetic studies to assess the effect of mating systems on the speciation process. In accordance with theoretical predictions, we find that accumulation of hybrid incompatibilities seems to be accelerated in selfers, but there is so far limited empirical support for a predicted bias toward underdominant loci. Phylogenetic evidence is scarce and contradictory, including studies suggesting that selfing either promotes or hampers speciation rate. Further studies are therefore required, which in addition to measures of reproductive barrier strength and selfing rate should routinely include estimates of demographic history and genetic divergence as a proxy for divergence time.