EVOLUTIONARY PROCESSES IN THE GENUS CAPSELLA (BRASSICACEAE)

Capsella comprises diploid (C. grandiflora and C. rubella) and tetrapl oid taxa. It is argued that the tetraploid C. bursa-pastoris is of int raspecific origin despite disomic inheritance and ''fixed heterozygosi ty''. It is of considerable age as evidenced by the fossil record and molecular data. Gene duplication by polyploidization and a mixed matin g system provided the genetic flexibility for greatest colonizing succ ess. Pronounced variation patterns at a micro- and macrogeographic sca le are observed in C. bursa-pastoris for many characters including lif e history traits, leaf morphology and allozymes. This variation patter n can be explained by several components comprising phylogenetic age, random processes, ecotypic variation and colonization history. The ada ptive strategy of C. bursa-pastoris cannot be assigned to either ecoty pic differentiation or phenotypic plasticity alone. It depends on the trait under study.