New species are usually formed by speciation - the act of an earlier species splitting into two. This takes place at a certain time and place. These species may then travel to new places. but they may be stopped from doing so due to mountains and seas, and by climate. Some species may never arrive at certain places. This means that two places with similar climate can often have different kinds of animals and plants. For example, marsupials which live in Australia, and animals which moved because of the Great American Interchange - which meant that there were very different species which came to live in North America and South America.