Biogeography, evolution and extinction
Biogeography is the study of the geographical distribution of plants and animals. Different continents are sometimes even islands are characterised by different species. Members of a species often become isolated into separate populations. This may be because they are isolated on an island (such as the species Darwin observed on the Galapagos Islands) or they may become ecologically isolated. Ecological isolation may be species that only live on mountain tops, lakes, or patches of forest surrounded by grassland.
A good example of geographical isolation is the Britain during the Last Ice Age and its isolation since. During the Last Ice Age there were up to 21 glacial and inter-glacial periods. The glacial periods saw much of the water in the oceans locked up in the thick ice at the poles. As a result, sea level fell and the British Isles were connected to Europe by land. The land bridge to Europe meant that plants and animals that could not usually get to Britain came and lived here. During some of the inter-glacials and after the end of the last Ice Age, about 10,000 years ago, most of the ice melted causing sea level to rise flooding the North Sea and English Channel. Species trapped on the British Isles after the seas rose developed in isolation from their European cousins.
The diversity of life on Earth today is far higher than at any point in its past. Increases in biodiversity have been hit by five mass extinction events such as the extinction 65 million years ago which wiped out the dinosaurs and several other major groups. The largest extinction event took place at the end of the Permian about 250 million years ago when 57% of marine life was wiped out. Extinction events are not all bad. After a mass extinction a wide range of habitats are left open for the surviving plants and animals to move into. New habitats mean that species adapt to their new environments and new species evolve. For example, a wide range of new mammal species evolved to fill the gaps left by dinosaurs after the extinction 56 million years ago.
Are we in the middle of a Mass Extinction? There have been five mass extinctions. Many scientists think that we are in the middle of another one. This time the extinction has been caused by humans.