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BIOGEOGRAPHICAL EVIDENCES OF EVOLUTION

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INTRODUCTION

Biogeography is the study of the distribution of animals and plants in the world. It is a subfield of geography that studies spatial patterns of diversity. ‘Bios’ means life, ‘geo’ means earth and ‘graphy’ means to write.

PANGAEA

Some scientists say that near the end of the Palaeozoic era which was about 250 million years ago, all the present-day continents formed a single large, irregular landmass called Pangaea. All the oceans were together called the Panthiaiassa Ocean. The landmass first split to form the Gondwana land and Laurasia. These eventually split up and the smaller landmasses floated over to their present positions. As the continents drifted away, the seas separated them, forming barriers to free intercontinental movements of organisms. Organisms evolved gradually in their different environments.

BIOGEOGRAPHICAL REALMS

The earth has been divided into six biogeographical realms on the basis of distribution of animals and plants. They are

1.Australian realm

2.Neotropical realm

3.Ethiopian realm

4.Orietal realm

5.Palearctic realm

6.Nearctic realm

->First proposed for birds by P.L.Sclater in 1857

->Proposed for animals by A.R.Wallace in 1876

->No relation to political boundaries

->Separated from one another by major barriers of seas, mountains or deserts.

DISCONTINUOUS DISTRIBUTION

Descendants of a common ancestor in some cases inhabit different continents and differ from each other. This is called discontinuous distribution.

Some examples

(i)Alligators – Alligators are present only in Southeastern USA and eastern China. The North American continent was connected with eastern Asia in the early Cenozoic by a land bridge in the Bering Strait region. The climate of the whole was much warmer than at present, and the alligators, as revealed by fossil evidence, were distributed over the entire region. In the later Cenozoic, the climatic changes, due to the ice age, caused the extinction of the alligators in most of the region. The south-eastern USA and Eastern China remained unaffected by glaciations, and here the alligators survived. As the alligators of the two regions have been separated for a long time, they have developed different random mutations. Therefore, they are high, but closely related species in the same genus.

(ii)Lung fishes – In the early stages of continental drift, South America, Africa, Antarctica, and Australia were interconnected. Lung fishes may have originated in the interconnected areas. Later, the continents got separated. Antarctica shifted to a faraway place. Lung fishes are now found, one species in South America, South Africa, and South Australia.

CONCLUSION

We saw the Pangaean theory, biogeographical realms, and discontinuous distribution in this session. In the next session, we can go deeper into the topics.

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