The Atacama Desert is thought to have formed by gradual desiccation from the Late Miocene onwards by cooling of the Humboldt Current. Subsequently, the uplift of the Andes created a rain shadow on their western foothills finally establishing hyper-arid conditions in the Atacama bassin during the Pliocene. The cooling of the HC also established a thermal inversion layer over the Pacific Ocean under which stratocumulus clouds are being trapped. The interception of these clouds with the coastal mountain range results in the formation of fog oases along the Pacific coast that host an astonishing diversity of plant life.
This biogeographic scenario offers the possibility to investigate life form evolution in a changing environment. The key ecological factors are expected to be (1) drougth stress through gradual desiccation, (2) habitat fragmentation through gradual desiccation, and (3) relieve of drought stress by formation of fog oases.
In this context I study the evolution of different life forms in the genus Oxalis. Oxalis is represented in the Atacama Desert with c. 20 endemic species. A few of them are cushion shrubs, while the majority has developed water-storing stems, leaves, and root tubers. Different species appear to have opted for either belowground or aboveground storage. Questions to be asked are: (1) Are the Atacama Oxalis a monophyletic group or not? (2) Is there a single origin of succulence or are there multiple origins due to convergent evolution? (3) Is there a tendency towards aboveground or belowground storage in the succulent taxa? To answer these questions a phylogeny is being constructed based on two chloroplast markers (trnL-L-F and psbA-trnH) sampled for Atacama Oxalis endemics and representatives of all west-Andean sections of Oxalis.