Describe in brief the biogeographical evidences in support of organic evolution.

Explanation

Biogeographical Evidences (Evidences from Biogeography)
Biogeography is the study of distribution of animals and plants. The evidences of evolution based on biogeography (G. bios-life, ge-earth, grapho-to write) are called biegeographical evidences.
Pangaea (Gr. all earth). It is believed that around the carboniferous period (about 345 million years ago) or slightly earlier, all the present-day continents were in the form of a single big land mass called pangaea. Later on, due to various geological changes, huge land masses broke off and drifted apart from one another. As these land masses (now called continents) moved away, they got separated from each other by the seas. The seas acted as barriers and prevented free movements of organisms among the continents. As these continents had different environmental conditions, so plants and animals evolved there were of different varieties.
Biogeographical Realms. The earth has been divided into six major biogeographic regions, called realms on the basis of distribution cf animals and plants. Dr. P.L. Sclater in the year 1858 proposed first time the division of the world into six realms or regions according to the distribution of birds. In 1876 A.R. Wallace adopted it for all the animals. These realms (regions) are:
(i) Palaearctic, (ii) Ethiopian, (ii) Oriental, (iv) Australian, (v) Nearctic and (vi) Neotropical.
Palaearctic and oriental realms are separated by high mountain ranges. Oriental realm and Australian realm are separated by Wallace line.
Evidences/for Evolution. Biogeographical evidences may be explained as follows.
1. Discontinuous Distribution. Sometimes closely similar species exit at widely separated places without any respresentative in intervening territory. This is called discontinuous distribution. Two specific examples of discontinuous distribution are given below.
(a) Alligators. They occur only in south-eastern United States and eastern China. The North (American continent was connected with east Asia in early coenozoic. The alligators were distributed over the entire region. But due to certain barriers, the alligators of two regions were separated for long time and developed some mutations. Therefore, these alligators are somewhat different but they are related species of the same genus.
(b) `"Magnolias"^(1), "Tulips"^(2)` and `"Sassafras"^(3)`. These plants now grow naturally in the eastern USA and in China only. The reason is the same as for the alligators.
2. Restricted Distribution. The parts separated from the main land have unique fauna and flora. For example, Australia has egg-laying and pouched mammals that occur only in Australia. This restricted distribution may be explained in the following way. Australia separated from the main land of Asia during mesozoic era, before placental mammals evolved. Placental mammals, being more adapted, eliminated the egg laying and most of the pouched mammals in other parts of the world. The egg laying and pouched mammals of Australia survived as placental mammals could not reach there due to lack of land route. e.g., Deserts of America have cacti while Africa have Euphorbia double coconut is found in Seychles island.