Senecio squalidus L. (Asteraceae) has been the subject of several ecological and population genetic studies due to its well–documented history of introduction, establishment and spread throughout Britain in the past 300 years. Our recent studies have focused on identifying and quantifying factors associated with the sporophytic self–incompatibility (SSI) system of S. squalidus that may have contributed to its success as a colonist. These findings are of general biological interest because they provide important insights into the short–term evolutionary dynamics of a plant mating system. The number of S–alleles in populations and their dominance interactions were investigated in eight wild British populations using cross–diallel studies. The numbers of S–alleles in British S. squalidus populations are typically low (average of 5.3 S–alleles) and the entire British population is estimated to possess no more than 7–11 S–alleles. Such low numbers of S–alleles are most probably a consequence of population bottlenecks associated with introduction and colonization. Potential evolutionary impacts on SSI caused by a paucity of S–alleles, such as restricted mate availability, are discussed, and we suggest that increased dominance interactions between S–alleles may be an important short–term means of increasing mate availability when S–allele numbers are low.