Distribution Historical Climate Change Changing sea levels Fluctuations in sea level have created and removed land bridges connecting continents and islands. This allows migration of species into areas previously unavailable to them, or conversely, isolates populations of the same species into discrete populations, which allows allopatric speciation to occur. Fluctuations in sea level occur as a result of glaciation. Sea level falls during ice ages, reducing the extent of warm inland seas, where much of the marine species live. Glaciers also affected current species distributions in another way.
Glaciers can also leave relict populations at high altitudes that remain ice free.
Rates of climate change can affect speciation. South America was subject to extreme climate changes during the Pleistocene and endured major alternations of wet and dry periods. Tropical rainforest may have undergone cycles of expansion and contraction. Many birds of tropical lowland forests are reluctant to cross ecological barriers so speciation by geographic isolation may occurred Current effects of climate on species distribution Current species distributions likewise are limited by the climate in the areas they inhabit. Some species are able to exist in a wide variety of habitats, while others may tolerate relatively low amounts of climate variation.