A Contribution of Rain Forest Research to Evolutionary Theory

Abstract

Though by no means universal even in the lowlands, species diversity within a single life-form reaches unequalled levels in many tropical forests, and in particular in the aseasonal wet oceanic climates of the Far East. There large genera, many of whose species may occur together and are apparently spatially interchangeable (e.g., Poore, 1968) are particularly frequent and have prompted speculation as to their origin. Ecologists (Poore) and taxonomists (Fedorov, 1966; van Steenis, 1969) alike have concluded that chance events are the major determinants of survival and must hence influence the course of evolution. Janzen's (1970) attractive theory that interactions between host-specific predators and their tree prey provide a density controlling mechanism which allows accretion of floristic diversity has yet to be investigated within large tree genera and does not apply within the Dipterocarpaceae, dominant trees of the Far Eastern rain forest canopy, whose predators are well known and are not specific even at generic level. How old are tropical tree species? How niche specific are they? Is evolution continuing within these forests? Are these communities in evolutionary equilibrium, following a long period of gradual stabilization (e.g., Stebbins 1974), or does species diversity continue to increase? What are the component tree species, are they outbreeders and are they genetically variable, or are they genetically uniform, even apomictic? Richards (1963) has reasoned that ecology cannot afford to ignore the tropics; an understanding of the evolutionary biology of this most species-rich vegetation must be accepted as equally essential if only to put, by comparison, knowledge of our younger and less diversified temperate counterparts into truer perspective. Studies in the Dipterocarpaceae and their forests over the last 20 years, in which I have collaborated, are beginning to elucidate this subject.