Evolution of endemic angiosperms of Tristan da Cunha

Research project P 14108 Evolution of endemic angiosperms of Tristan da Cunha Michael KIEHN 11.10.1999 The project deals with the evolution of endemic angiosperms of the isolated oceanic archipelago, Tristan da Cunha. The objective is to answer general questions regarding speciation and other evolutionary processes on islands as well as on continents. The Tristan group consists of three islands, situated in the central South Atlantic Ocean. The islands are of volcanic origin and of different geological ages, ranging from 0.5 to 18 million years. Previous botanical work has provided a good inventory of taxa as an important basis for the proposed study. Tristan represents an outstanding island system for evolutionary studies. It is one of the most isolated places in the world, and the vegetation is, compared with most other oceanic island, only slightly disturbed by humans. The endemic flora appears to be in an early stage of evolution, which stands in sharp contrast to the geographical isolation and the age of the oldest island of the group. The investigations will focus on four selected genera. This group covers taxonomic diversity (four families) as well as different evolutionary modes: anagenesis in Ranunculus, cladogenesis in Acaena, Nertera and Cotula. These generic systems will be used to investigate evolutionary change taking place between progenitor and derivative taxa. Studies on adaptive radiation within the archipelago will also be carried out in Agrostis, with six endemic species. The principal questions of the proposed investigation are: (1) What changes in morphological character states have occurred during sepciation, and are they common to all endemic taxa? (2) What changes in chromosome numbers and/or karyotypes have occurred during speciation? (3) What type and degree of genetic change have occured within each evolutionary line from continental progenitor to island derivative species? (4) How do molecular clock measures coincide with known geological ages of islands in the archipelago? (5) What levels of morphological and genetic variation exist within and among populations of endemic taxa, particularly in comparison to continental progenitors? (6) What have been the processes of speiation within the archipelago? As a final step we will summarise results on patterns of evolution during speciation on Tristan and compare them with results from other islands. We hope also to understand why the endemic flora is not as divergent as might be expected based on age and isolation of the archipelago.

Tristan da Cunha is an isolated volcanic archipelago in the South Atlantic, consisting of four islands. With their endemic species (species only occurring there) such archipelagos are good model systems to study evolutionary processes. In the course of two expeditions (five month, 2000 and 2001) to the islands Tristan da Cunha, Inaccessible, and Nightingale material for molecular, karyological and morphological research was collected to analyse phytogeographical and evolutionary pattern of selected species and genera. Results examplified for the genus Nertera (Rubiaceae) This genus occurs on Pacific and South Atlantic islands as well as on the Asian mainland, in Central and South America. It comprises 11 species, six of which are found in New Zealand. The only widespread species is N. granadensis. Besides this species (extant on all four islands) the Tristan da Cunha archipelago harbours two endemic species: N. assurgens and N. holmboei (both found on two islands). The following questions were originally adressed: (a) what is the geographic origin of the species from Tristan; (b) how well are the species on the archipelago differentiated geographically, morphologically and genetically; and (c) how are they related to each other? Besides Nertera-collections from Tristan material of all other described species was analysed (e.g. from Meso- and South America, from Asia, New Zealand and Hawaii). The results exceed the original scope of the investigations: (a) the genus seems to have evolved in New Zealand and then spread into the Atlantic area and to South America; (b) N. granadensis, considered to be morphologically homogenous throughout its distribution range, is genetically diverse; a split into several distinct taxa could be considered; (c) populations in America are derived from two independent long distance dispersal events; only the southern group (from Chile and Argentina) seems to be directly related to the New Zealand populations of N. granadensis; (d) all Tristan da Cunha populations are the result of a recent single colonisation event from southern South America; and (e) the genetic differentiation amongst the Tristan da Cunha populations of N. granadensis and to the two endemic species (N. assurgens and N. holmboei) is weak and could indicate the recent development of the morphologically well defined endemics. Thus the results are also relevant for the question of the definition of a species as they show a discrepancy between the actual species concept (based on morphology) and the delimitation inferred by the genetic differentiation.