‘The formation and divergence of species’ is concerned with the evolution of new species and of differences between species. Different species that coexist in the same locality remain distinct because they do not interbreed. This lack of interbreeding is called reproductive isolation and the ways in which closely-related species are prevented from interbreeding are described. The tendency of populations of the same species to differ according to their geographical location, often in ways that are clearly adaptive, is called geographical variation. Molecular evolution and divergence between species is also discussed with information from DNA and protein sequences allowing biologists to make inferences about the relationships between different species.