Evolution in the Tropics

The substance of this article was delivered as the Presidential Address to the Society for the Study of Evolution, December 30, 1969, in Boston, Massachusetts. Attention is drawn to evolutionary studies in the tropics which appear to be advancing the subject significantly. Then, detailed attention is given to the possible evolutionary bases of the most striking biological characteristic of the tropics: the extraordinary floristic and faunistic diversity of tropical ecosystems. Because most contributions to the discussion of this topic have come from zoologists, the emphasis here is placed on botanical aspects. It is concluded that each of several potential explanations for the diversity may be valid and that a great need for the future is synthesis rather than arbitration between theories. THERE ARE MANY reasons why, as we come to the end of the 1960's, we should pay attention to what is happening in the study of evolution in the tropics. After many years of neglect, the tropics are getting more study by ecologists-and where the ecologists find their stimulation and their information there is also material for evolutionists, provided that we are still interested, as they are, in adaptation. Despite recent claims by molecular biologists (e.g., King and Jukes 1969, Wilson and Sarich 1969) for nonselective biochemical evolution, I believe that adaptation, maintained by natural selection, is still the cornerstone of the evolutionary edifice-and the tropics provide superb opportunities for studying multifarious examples of adaptation in action. Quite recently there have been important advances, with evolutionary significance, in several areas of tropical biology, and I should like to draw attention to some of them. For example, one of the most exciting developments in plant physiology in the last few years has been the demonstration that there are at least two biochemical pathways along which photosynthesis by flowering plants may proceed. The conventional or Calvin pathway (Calvin and Bassham 1962) involves phosphorylated chemical compounds with three carbon atoms while the more recently discovered pathway (Hatch and Slack 1966) involves four-carbon, dicarboxylic acids as early products of photosynthesis. Plants using the four-carbon pathway can fix carbon dioxide at rates up to twice that of those using the conventional mechanism, at least partly because they waste nothing in photorespiration. Correspondingly, their carbon dioxide compensation points (the level of CO2 in the atmosphere at which respiration just balances photosynthesis) are extremely low. Furthermore, they have peculiarities in the bundle sheaths of the leaves (Laetsch 1968) by which they may be recognized, as well as leaf structures which reduce gaseous exchange. Accumulation of carbon dioxide is facilitated under certain dark circumstances and, because C4 photosynthesis is not inhibited by accumulating oxygen during illumination (Olle Bjorkman, pers. comm.), their photosynthetic rates continue to increase as light intensities up to the maximum experienced in daylight are reached. Photosynthesis also continues even when dissolved carbon dioxide concentrations in the cells are reduced to low levels