Evolution and speciation in a tropical high mountain flora

Abstract

Few ecosystems provide better opportunities for the study of evolution and speciation than those inhabiting the uppermost parts of the high east African mountains. These mountains are mainly of volcanic origin and lie widely scattered across the wide plateaux of east Africa, several of them reaching altitudes between 3500 and 6000 m. Their vegetation deviates very much from that of the intervening lower country, displaying a marked zonation with a montane forest belt, a (subalpine) ericaceous belt, and an afroalpine belt. The flora of the latter, the afroalpine flora, is of exceptional interest in this connection. The afroalpine flora is famous for its large numbers of geographically vicarious taxa–its Giant Senecios and Giant Lobelias are as renowned as the finches of the Galapagos Islands. Ecologically, the afroalpine biota is indeed also an island biota–the high mountain summits protrude as isolated temperate islands above the warm surrounding plains. These mountains have evidently stood isolated from each other since their origin. Pleistocene climatic changes have certainly modified their vegetation zonation to a considerable extent, but direct contacts between their afroalpine enclaves during the Pleistocene or earlier seem most improbable. These enclaves must therefore have been isolated from each other and from other high mountain areas for a very long time, and dispersal of plants between them must presumably have occurred mainly by long distance transport, possibly facilitated by cyclones. Some 80% of the afroalpine species of vascular plants are endemic to the high mountains of tropical east Africa and Ethiopia. Vicarious taxa occur of different status. In some cases one species occurs on all or most of the east African mountains with a vicariad in other parts of the world, as exemplified by Subulariamonticola (afroalpine) and S. aquatica (circumboreal). In other cases each of two species is confined to one group of mountains, as in the species pair Lobelia wollastonii (Virunga Volcanoes and Ruwenzori) and L. telekii (Elgon, Aberdare, Mt Kenya). Finally, there are several groups of vicarious taxa where each taxon is confined as a rule to a single mountain, as in the Lobelia deckenii group with six cognate species. Numerous similar cases occur among spiders and insects. The differentiation of these vicarious species is assumed to have occurred through natural selection in connection with genetic drift, acting upon geographically isolated and originally very small random samples of the gene pools concerned. The amount of differentiation between different mountain populations differs considerably between different groups–numerous intermediate stages exist between morphologically indistinguishable populations and full-fledged vicarious species. The rate of evolutionary change seems to differ considerably between different genera and families. Whether internal barriers to interbreeding exist between these vicarious taxa is in most cases unknown. No less interesting than the geographically vicarious taxa mentioned above are some cases of altitudinal vicariism. Some afroalpine species appear to have evolved from afromontane forest species through progressive adaptations favouring survival in the inhospitable afroalpine climate. The most remarkable examples are provided by the strangely specialized Giant Senecios and Giant Lobelias.