Similar species of pests and diseases attack crops worldwide. The diversity of plants differs between the New and Old World. Natural selection acts on differences between populations. Evolution by natural selection is a very slow process. Evolution cannot occur in populations made up of different clones.
Natural selection acts on differences between individuals within populations.
Evolution occurs because individuals can evolve. Less than one percent of the genes sampled were variable. The population has almost no genetic variation. The population is experiencing balancing selection. Barrett's work on tristyly in Water Hyacinth reported in Scientific American indicates that this polymorphism does not function effectively as an outcrossing mechanism, especially in the introduced range of the species.