Climate affects taxonomic similarity of regions, but only partly. The fossil record provides evidence of past distributions. Competition, predation, and facilitation influence distribution, dispersal, and extinction. Speciation occurs through geographic separation when isolated populations diverge in their adaptations to different climates and habitats. Long-distance dispersal is possible, and is probably the only means of colonization of oceanic islands.
Some taxa are better at it than others.
Trans-oceanic distributions of taxa that are not good dispersers are evidence of past land connections. In the absence of predators or competitors, organisms on isolated land masses evolve to common forms. When two land masses are connected and exchange biota after long separation, extinction of organisms by new competitors may result. The processes observed today may be different in intensity than those processes in the past. Island biotas are important because they provide clearer evidence of speciation, adaptation and distribution.