In this thesis, broad phenotypic variation in the genus Alexandrium with respect to allelochemical properties targeting a wide range of other planktonic protists was observed and indications of co-evolutionary processes shaping the respective allelopathic phenotype were found. At the intra- and inter-population level in the species A. tamarense large genotypic variation was observed. A conceptual population genetic model on how this variation is preserved in the long run - despite evidence of clonal selection and frequency shifts of clonal lineages during vegetative planktonic growth - is presented. Phenotypic characters such as allelochemical properties and content and relative composition of paralytic shellfish poisoning toxins (PSP) may be such characters that are under directional selection during planktonic population growth and harmful algal bloom (HAB) development. However, isolates from the populations studied varied widely with respect to these quantitative genetic characters
