Parallel Evolution in African and Mascarene Hypericum

Abstract

Summary. The East African Hypericum keniense is distinct from H. angustifolium of Reunion, but each species has a closely related derivative taxon which differs from it by similar characters, respectively H. revolutum and H. lanceolatum. These pairs of taxa are regarded as only subspecifically distinct (the appropriate nomenclatural changes being made); and they are shown to have evolved from an ancestral species that probably occupied a single area in the East Africa-Madagascar-Mascarene region in the Cretaceous era, before these land blocks were separated by rift and drift. The species of Hypericum that have the greatest number of primitive characters belong to sect. Campylosporus, a section that is represented in tropical and south-eastern Africa, Madagascar, the Comoro Islands, Reunion, Socotra, south-eastern Arabia and Fernando Po (Robson, 1977). Within this section the most primitive species appears to be H. bequaertii De Wild., which is confined to the Ruwenzori Mountains of Uganda and Zaire (Milne-Redhead (i953), Bamps (1970)), where it grows in the upper part of the ericaceous belt and the lower part of the alpine belt (Hedberg, 1957). Very closely related to it is the taxon known hitherto as H. keniense Schweinf., which occurs at somewhat lower altitudes on several of the East African mountains, including Ruwenzori (Ross (1955)), where its zone of occurrence (from the montane forest belt to the alpine belt) overlaps that of H. bequaertii. The species are, however, ecologically distinct, as H. bequaertii inhabits moist and protected localities whereas H. keniense grows on comparatively dry, more open ground. This evolutionary series is continued with H. revolutum Vahl, which is present in the montane forest belt at lower altitudes than H. keniense but overlaps it in range. It is characteristic of the uppermost zone of this belt, together with Hagenia abyssinica (Bruce) Gmelin (Rosaceae; Hedberg (1950)). Its distribution, however, is much wider than that of H. keniense and includes almost the whole of the mainland area of sect. Campylosporus, i.e. except the extreme west (Nigeria, Guinea), the extreme northeast (Somalia, northeastern Ethiopia) and Angola; but it is absent from the islands except Fernando Po (Map 2). This series of species ends with H. gnidiifolium A. Rich., which has a restricted distribution in the Tigre and Shoa Provinces of Ethiopia. Its foliar, inflorescence and floral characters can be easily derived from those of H. revolutum, but not from those of H. roeperanum Schimper ex A. Rich., of which Moggi & Pisacchi (1967) considered it a subspecies. In 1797 Lamarck described from the island of R6union two species,