Biogeography
New species are usually formed by speciation  an earlier species splitting into two. These species may travel to new places. but they may be stopped from travelling by mountains and seas, and by climate. This means that two places with similar climate often have different kinds of animals and plants. For example marsupials, which live in Australia, are very different from the fauna in South America. The species on islands (Hawaii, Galapagos) may be very different to species on mainland continents.