Changes in breeding system are a regular evolutionary change in plants, as self-fertilisation is often advantageous, particularly for weedy and colonising species. The adoption of Arabidopsis thaliana as a plant model species has led to interest in how self-incompatibility was lost so that this species became highly inbreeding. Molecular evolutionary approaches have recently focused on investigating two loci involved in the incompatibility recognition process in related Arabidopsis species; non-functional copies of these genes still exist in A. thaliana. New work studying polymorphism at these loci found strikingly low diversity at one of them, suggesting that spread of a mutation in this gene might have caused self-compatibility in an ancestor of A. thaliana.1 However, it is difficult to be sure of the time when the selfing habit evolved in the lineage that led to A. thaliana BioEssays 27:472–476, 2005. © 2005 Wiley Periodicals, Inc.