Plant speciation in Malesia, with special reference to the theory of non-adaptive saltatory evolution

Tropical genera are frequently old, several going back to at least the Upper Cretaceous. Extinction and subsequent disjunction of range is a rather common phenomenon, the reasons for which are obscure. Tropical rain-forest groups have carried important potentialities and this has led to the development from these ancestral groups towards many specialized microtherm offshoots peopling the colder parts of the globe. This has sometimes also led to north-south bi-hemis-pherical different counterpart families, affinities, or lineages. Evolution among tropical plants has worked in an erratic way and there is an astonishing variation in the rate of potentialities. This cannot be correlated with degree of versatility in adaptation. The generous tropical rain-forest conditions offer the lowest possible survival-value level to plant development, i.e. the lowest competition intensity. For plants of the undergrowth there may be a minor limitation set by pollination deficiency. There are roughly three main kinds of species in the tropical rain forest, (i) the coenospecies, with a variant the complex species, (ii) the reticulately allied species forming ‘solid blocks’, and (iii) the isolated species of genera of remote alliance. The coenospecies and complex species came into being through development by a very gradual process of differentiation by small changes during a long period of dispersion. They have probably not led to the origin of the solid blocks of which the species are often sympatric. Both categories belong to micro-evolutionary change, but did not contribute to macro-evolution. The tropical rain forest harbours an unusually large number of isolated species and an impressive number of unusual structures. Many of these are taxonomic parallels of terata and all can be derived from growth phenomena. Similar abnormal phenotypes, with considerable change in high-ranking structures, may be caused by sudden genetic changes, often single gene mutations. Also synthesogenic changes are sudden steps (amphi- and aneuploids). The low survival level of the tropical rain forest has offered the best opportunities for the survival, preservation, and further development of such saltatory evolution. Terata can be arranged in a few main groups according to growth phenomena and there is a striking parallel between terata and taxonomical monstrosities. From this correlation it appears plausible that the ‘hopeful’ monsters have been an, or may be even the most, important means of macro-evolution by single large steps. With the length of geological time and the infinite numbers of individual tropical plants, an infinitesimally small number of nature's experiments succeeding could well have led to attaining new supraspecific structural levels by saltation. After all the number of structural plans is definitely restricted. Data on hybridization are hitherto scarce and polyploids are scant in the ligneous families; both impressions may be premature through lack of sufficient research.