Significance

Scientists have long debated why some regions of the Earth, such as the tropics, have more species than other regions. We tested all of the major hypotheses simultaneously and came up with an answer — time. In other words, groups of organisms that have occupied areas longer have more species because they have had more time to produce them. This conclusion goes against the prevailing thought in the fields of ecology and evolution that ecological factors—such as the interactions of species and their environment—primarily determine the diversity and distribution of species around the globe.