When the African and American continents separated 65125 million years ago, populations and communities of plants and animals of the same lineage became isolated and evolved independently. Today, however, the cyprinodontiform fishes on the two sides of the Atlantic Ocean are morphologically similar, especially the American genus Rivulus and the African genus Aphyosemion. The evolutionary-inertia hypothesis is rejected as an explanation of this similarity because of evidence that the speciation process is ongoing. This study concerns processes that are probably responsible for the convergent and divergent evolution in the composition, structure, and dynamics of populations and communities, respectively. Basic data were obtained during a long-term study in the field, at the M'Passa biological station in Gabon and the St. Elie biological station in French Guiana. At the same time, rearing experiments were conducted from the Laboratoire d'Écologie at the Muséum National d'Histoire Naturelle in Paris. The composition, structure, and dynamics of the populations at M'Passa and St. Elie are similar. Parallel evolution of the habitats may explain the parallel evolution of the populations of cyprinodontiform fishes in Africa and America, leading to similar fish populations in similar habitats. Conversely, the composition, structure, and dynamics of the communities at St. Elie and M'Passa are different. The cause of this divergent evolution may be the absence of specific predators and competitors at M'Passa and the presence of highly specialized predators and competitors at St. Elie. The predators and competitors present at St. Elie all belong to the Characidae, a very successful group of fishes in America, without an equivalent in the Old World. Analogous processes of competitive exclusion are also observed in other orders of tropical American vertebrates: batrachians, reptiles, birds, and bats.