Marshall L G, Webb S D, Sepkoski J J, Raup D M
Science. 1982 Mar 12;215(4538):1351-7. doi: 10.1126/science.215.4538.1351.
A reciprocal and apparently symmetrical interchange of land mammals between North and South America began about 3 million years ago, after the appearance of the Panamanian land bridge. The number of families of land mammals in South America rose from 32 before the interchange to 39 after it began, and then back to 35 at present. An equivalent number of families experienced a comparable rise and decline in North America during the same interval. These changes in diversity are predicted by the MacArthur-Wilson species equilibrium theory. The greater number of North American genera (24) initially entering South America than the reverse (12) is predicted by the proportions of reservoir genera on the two continents. However, a later imbalance caused by secondary immigrants (those which evolved from initial immigrants) is not expected from equilibrium theory.
大约300万年前，在巴拿马陆桥出现之后，南北美洲之间开始了陆地哺乳动物的相互且明显对称的交换。南美洲陆地哺乳动物的科数量从交换开始前的32个增加到交换开始后的39个，然后目前又回到35个。在同一时期，北美洲相当数量的科也经历了类似的增减变化。这些多样性变化是由麦克阿瑟-威尔逊物种平衡理论预测的。根据两大洲源库属的比例预测，最初进入南美洲的北美洲属（24个）比反过来进入北美洲的南美洲属（12个）更多。然而，平衡理论并未预测到后来由二次移民（即从最初移民进化而来的那些）导致的不平衡。
