Selfing has evolved in animals, fungi, and plants, and since Darwin’s pioneer- ing study, it is considered one of the most frequent evolutionary trends in flowering plants. Generally, the evolution of selfing is characterized by a loss of self-incompatibility, the selfing syndrome, and changes in genome-wide polymorphism patterns. Recent interdisciplinary studies involving molecu- lar functional experiments, genome-wide data, experimental evolution, and evolutionary ecology using Arabidopsis thaliana, Caenorhabditis elegans, and other species show that the evolution of selfing is not merely a degradation of outcrossing traits but a model for studying the recurrent patterns underlying adaptive molecular evolution. For example, in wild Arabidopsis relatives, self- compatibility evolved from mutations in the male specificity gene, S-LOCUS CYSTEINE-RICH PROTEIN/S-LOCUS PROTEIN 11 (SCR/SP11), rather than the female specificity gene, S-LOCUS RECEPTOR KINASE (SRK), sup- porting the theoretical prediction of sexual asymmetry. Prevalence of domi- nant self-compatible mutations is consistent with Haldane’s sieve, which acts against recessive adaptive mutations. Time estimates based on genome-wide polymorphisms and self-incompatibility genes generally support the recent origin of selfing.