In the evolution of floral structures a number of examples exist where homeotic changes in flower organisation probably occurred at the base of taxonomic groups. It is, however, not known how exactly natural morphological diversity is generated, especially how homeotic changes which originated spontaneously in the wild get fixed in a population. A plant model system in which the role of a spontaneous homeotic mutation can be studied in a multidisciplinary approach combining morphological, genetic, molecular and ecological methods was not available so far. In this thesis I established the natural floral homeotic Spe (Stamenoid petals) variety of Capsella bursa-pastoris, in which petals are completely transformed into stamens, as such a model system. In a concomitant review article known facts about the role of homeosis were compiled to demonstrate the potential of C. bursa-pastoris as a model species and for introduction of the Spe variant. A hypothesis on the underlying molecular mechanism based on ectopic expression of a class C organ identity gene as well as an experimental program to study this phenomenon with ecological and evolutionary methods was developed in the review article. With my experimental results I demonstrated that the Spe variety shows a complete homeotic shift from petals to stamens that is driven by an allele of a single co-dominant locus in two different Spe variety lines without pleiotropic effects. The co-dominant mode of inheritance of the Spe phenotype suggested a gain-of-function mutation in a regulatory region of the same gene in two independent Spe variety lines. I also showed that ectopic expression of two AG-like genes, CbpAG and CbpSTK, correlate with organ transformation, of which very likely the CbpAG ectopic expression is directly responsible for the changed organ identity. Results of parallel PhD work of J. Ziermann demonstrated that exclusively the class C gene CbpAGa co-segregates with the mutant phenotype and that likely a change in a candidate cis-regulatory region of the CbpAGa gene is responsible for the deviating expression patterns of this gene. These results have been included into the publication of my analyses.