Evolutionary processes in the genus Capsella (Brassicaceae)

Abstract

Capsella comprises diploid (C. grandiflora and C. rubella) and tetraploid taxa. It is argued that the tetraploid C. bursa-pastoris is of intraspecific origin despite disomic inheritance and `'fixed heterozygosity''. It is of considerable age as evidenced by the fossil record and molecular data. Gene duplication by polyploidization and a mixed mating system provided the genetic flexibility for greatest colonizing success. Pronounced variation patterns at a micro- and macrogeographic scale are observed in C. bursa-pastoris for many characters including life history traits, leaf morphology and allozymes. This variation pattern can be explained by several components comprising phylogenetic age, random processes, ecotypic variation and colonization history. The adaptive strategy of C. bursa-pastoris cannot be assigned to either ecotypic differentiation or phenotypic plasticity alone. It depends on the trait under study.