Isozyme variation in tropical trees: patterns of genetic organization

Tropical forests have long been of interest to biologists because of their high species diversity and their complicated patterns of community organization. The traditional notion of the rain forest as an ancient, unvarying archive of species has been challenged by a host of recent ecological studies which have demonstrated that tropical trees are diverse in their reproductive biology and dynamic in their population structure.Data from 97 isozyme studies on the genetics of tropical woody species demonstrate that cultivated taxa maintain higher percentages of polymorphic loci and higher mean heterozygosities than native tropical species. Levels of within-population variation in tropical taxa are as high or higher than in plants in general, previously reviewed by Hamrick and Godt (1989). Levels of genetic variation differ significantly among species with different geographic ranges, life forms, and taxonomic affinities. Levels of population differentiation, measured by Gs, values, are significantly different only between species with different seed dispersal modes. Outcrossing rates in 16 tropical tree species showed a preponderance of highly outcrossed breeding systems. Genetic evidence suggests that gene flow among local populations is high, but geographically separated populations show moderate levels of genetic differentiation.Ecological and historical processes provide the mechanisms influencing the genetic architecture of plant species. Where appropriate, data on isozyme variation in tropical trees are related to breeding systems, seed dispersal mechanisms, demography, and patterns of environmental heterogeneity. Profitable avenues for future research are suggested.