INFLUENCE OF THE DURIAN THEORY

Overview Corner and the durian theory Ashton, Fedorov and evolutionary processes Gentry's hypotheses Applications     • pollination     • breeding systems     • allozyme analysis     • architecture  References and further reading  Questions

Corner (1949, 1954) stimulated continued intense debate and field investigations into evolutionary processes in tropical forests following his proposal of the Durian Theory. This view of the evolution of tropical forest was based upon observations of a series of unusual features of tropical plants.  Such features included  fruits with a fleshy aril  fruits displaying distinct black and red colouration  seeds hanging away from the fruit  Put your mouse cursor over the image to see a close up of the Sterculia sp. fruit.    These features were relatively rare in forest plants as a whole, but occurred across a broad range of plant groups in the tropics.  Corner's description of the observations which led him to formulate the Durian theory is deceptively simple but developed as he expanded his hypotheses to encompass a greater range of phenomena across the tropics. His ability to link a series of apparently unrelated characters with changes inherent in developmental processes allowed him to successfully project this onto an evolutionary framework.  Why would such obscure features appear in a number of apparently unrelated groups?  Corner postulated that these were ancestral characters, which remained as anomalies in present day forest but which, put into an evolutionary context, could be used to explain the development of complexity and plant diversity in tropical forests.      The starting point was a large fruit similar to the durian Durio zibethinus (Bombacaceae). This possessed a distinct coloured aril surrounding the seeds, was heavily armoured and massive in structure. The seeds of such a fruit would be dispersed by a variety of animals attracted by the nutritious fleshy aril. A massive fruit would require a massive tree on which to be carried, probably something like the present day palms or cycads. Thus the durian theory postulated an origin for angiosperms from a cycad/palm like ancestor rather than the more delicate gymnosperms such as conifers.   Put your mouse cursor over the image to see a close up of the durian fruit.    This ancestral tree grew from a single axis, as do most palms today, with terminal leaves and a single terminal inflorescence. However, this growth form provides little potential for development to give the variety of form now present in forests. The next evolutionary development therefore allowed for the production of branches.  As branching systems arose there was an overall reduction in the size of component parts ie branches, leaves and flowers as the whole plant or tree became progressively larger.  Increase in size and longevity of such trees, in part due to the removal of the limitations inherent in the possession of terminal inflorescences, meant that diversity of form also increased. This diversity was accompanied by an associated increase in animal diversity, as animals diversified to live in canopy environments as opposed to being ground dwelling, and began to feed on drier and smaller fruits and seeds.  Over time this ongoing process could give rise to all facets of the complexity seen in the present day tropical forests, and would also explain the changes in plant form required to survive outside the tropics in adjacent seasonal temperate areas.  Although this model cannot be tested it has stimulated much current research into evolutionary mechanisms in the tropics and remains a significant milestone in the study of tropical biology.  NOW TEST your understanding of the basic principles by answering some questions.  Preliminary questions  Extension questions