Speciation among tropical forest trees: some deductions in the light of recent evidence

It has often been supposed that tropical rain forest, owing to its complexity, poses a unique situation for the processes of outbreeding and speciation. Fedorov (1966) has recently summarized traditional notions on the subject and provided a new interpretation. The present paper discusses Fedorov's views and endeavours to show that our present knowledge of rain-forest ecology does not necessitate formulation of modes of speciation essentially different from those within other terrestrial plant ecosystems. After a discussion of the implications of genetic drift in the light of present knowledge, evidence is presented that tropical tree taxa, as exemplified by the species-rich south-east Asian Dipterocarpaceae, possess the following characteristics: 1 Highly specialized adaptation to their biotic and physical environment, whether or not a single niche may be filled by one of several species in part by historical accident. 2 Limited efficiency of fruit dispersal imposing contagious distribution of individuals within their habitats and inability to cross all but the narrowest dispersal barriers. 3 An unspecialized pollination system in which autogamy is usual, but outcrossing between individuals of a clump, and to a lesser but significant extent between clumps of a population, frequent enough to allow gene exchange throughout populations in a continuous habitat. 4 Allopatric differentiation between populations in response to differential selection pressures. 5 Rarity of fertile hybrid populations; hydridization is often prevented owing to differentials in flowering time. 6 Great morphological constancy even within taxa with a widespread but disjunct distribution; rarity of clinal variation. 7 Long life-cycles combined with low numbers, but high numerical constancy, of chromosomes. The complexity of the rain forest can be explained in terms of: (i) The seasonal and geological stability of the climate which had led to selection for mutual avoidance, and, through increased specialization, to increasingly narrow ecological amplitudes, leading to complex integrated ecosystems of high productive efficiency. As the complexity increases, the numbers of biotic niches into which evolution can take place increase but become increasingly narrow. (ii) Their great age. It is concluded that decreasing ecological amplitude has been accompanied by a reduction in the gene pool of constituent species, explaining their morphological uniformity and exceptional stability. The initial, fundamental, and apparently non-adaptive, diversity by which the many rain-forest tree families are characterized could have arisen when the early angiosperms invaded and diversified, presumably allopatrically, in what was to them a vacant humid tropical environment occupied by gymnospermous forest; diversification on this scale has become increasingly rare as the ecosystem evolved, and has subsequently occurred only, as in the case of the dipterocarps, when a group invades an extensive ecological niche previously not or little exploited.