Formation of the Isthmus of Panama.

The formation of the Isthmus of Panama stands as one of the greatest natural events of the Cenozoic, driving profound biotic transformations on land and in the oceans. Some recent studies suggest that the Isthmus formed many millions of years earlier than the widely recognized age of approximately 3 million years ago (Ma), a result that if true would revolutionize our understanding of environmental, ecological, and evolutionary change across the Americas. To bring clarity to the question of when the Isthmus of Panama formed, we provide an exhaustive review and reanalysis of geological, paleontological, and molecular records. These independent lines of evidence converge upon a cohesive narrative of gradually emerging land and constricting seaways, with formation of the Isthmus of Panama sensu stricto around 2.8 Ma. The evidence used to support an older isthmus is inconclusive, and we caution against the uncritical acceptance of an isthmus before the Pliocene.

Latitudinal difference in biodiversity caused by higher tropical rate of increase.

Tropical diversity has generally exceeded temperate diversity in the present and at points in the past, but whether measured differences have remained relatively constant through time has been unknown. Here we examine tropical vs. temperate diversities from the Neogene to Recent using the within-habitat diversity measure Fisher's alpha of Cenozoic benthic foraminifera from the temperate Central Atlantic Coastal Plain and the tropical Central American Isthmus. During the Neogene, the mean value of alpha at temperate latitudes increased 1.4 times or 40%, whereas in the tropics it increased 2.1 times or 106%. Thus, while both areas exhibit an increase of diversity with time, past differences in the rate of increase have generated a more pronounced gradient today (164%) than existed in the Miocene (80%). These data disagree with the suggestion that the world reached an equilibrium number of species during the Paleozoic and demonstrate the need to consider both temperate and tropical components in global diversity assessments.