One of the most important questions in science is what determines species diversity. Monanthotaxis is a diverse flowering plant group of lianas occurring in the tropical rain forests of Africa. However, the species of this group are very difficult to identify and it is unknown why there is so much species diversity in the African tropical forests. A few species have cancer-inhibiting properties and some representatives, including some unidentified, are locally used against malaria. My thesis lays the foundations that will facilitate any future research on this group. Twenty new species for science were discovered, described and named in this thesis and the evolutionary relationships between the species were resolved. The flowers of Monanthotaxis are also very diverse and we have shown a reduction of flower size and complexity in time. It is demonstrated that different pollination systems have generated the enormous diversity of this plant group.