Land bridge

A land bridge is a term in biogeography. It is a narrow land connection between otherwise separate areas, sometimes called an isthmus. 
Its significance is that animals and plants may cross over it when the sea level is lower. They cannot easily cross when the sea level is higher, and water covers the land bridge. The separation of a once connected population often leads to speciation, the splitting of one species into two and the two species change over time to be different but have distant similarities.
Several kinds of change can cause a land bridge to rise above or sink below the water. Plate tectonics may cause the rise or lowering of land. The causes of general sea level changes are not fully understood, but often big polar ice caps lower sea level by locking up much of the water. When sea levels fall, shallow sections of continental shelf may get exposed. Sometimes the sea floor rises due to "post-glacial rebound" after an ice age.