First thoughts on species evolution in Malayan Macaranga (Studies in Macaranga III)



Macaranga (Euphorbiaceae) is a genus of tropical trees. Five aspects of species evolution in Malaya are discussed. (1) Seventy per cent (20) of the species in Malaya have come to be common as small trees in secondary forest. Their original habitat was small forest clearings mainly along rivers, where they grow as big trees in small populations, fruiting all the year round. Their biology has enabled them to become the most successful genus in the extensive secondary forests that have resulted from forest clearing over the last 90 years. In so spreading they have not hybridized or developed regional differences within the country. (2) There are however marked differences from populations in northern Borneo, 800 km east over the shallow South China Sea; this is thought to reflect morphological divergence since the lands were last separated in the Pleistocene. (3) M. andamanica has a disjunct distribution coupled with polymorphism. (4) M. laciniata which replaces M. heynei in north and east Malaya has trivial morphological differences, thought to be genetically simple. (5) M. quadricornis, member of a close-knit species group round M. triloba, grows in lowland swampy forest in south Malaya to 2o20'N and in the mountains of central and north Malaya from 3o20'N. It is a very conspicuous and common species yet no intermediates have been found between these two groups. The simplest explanation of the disjunction in habitat and distribution seems to be that it has evolved twice from out of its complex.