An engine of evolution
The Peak Wilderness Sanctuary, a UNESCO World Heritage site in Sri Lanka's central highlands, demonstrates adaptive radiation. This is an evolutionary process where organisms rapidly diversify from a common ancestor. In these mountains, the primary species in this process are the shrub frogs of the genus Pseudophilautus. The ancestors of these frogs are thought to have migrated from India around the Eocene-Oligocene boundary, and in the isolated, stable environment of Sri Lanka's wet highlands, they diversified into a vast array of species.
The terrain itself is the source of this diversity. The sanctuary consists of a series of cool, wet mountain ranges separated by warmer, drier valleys. These "sky islands" isolate frog populations, preventing gene flow and allowing them to evolve independently. Over millions of years, this isolation has resulted in extreme specialization. Researchers have identified different "ecomorphs"—frogs adapted to specific microhabitats like leaf-litter, rock boulders, and high canopies. This intense speciation means that of the more than 120 amphibian species in Sri Lanka, about 90% are endemic, found nowhere else on Earth. Many of these are "point endemics," with ranges restricted to a single mountainside or forest patch.
Life without water
An important evolutionary innovation that allowed shrub frogs to colonize these mountains is direct development. Most Pseudophilautus species completely bypass the free-swimming tadpole stage. Females lay eggs in moist soil, leaf litter, or on leaves, and these hatch directly into fully-formed miniature frogs. This reproductive strategy frees them from the need for standing water, allowing them to thrive in the damp cloud forests.
This biodiversity is extremely fragile. Because many species have such tiny ranges, they are vulnerable to extinction. The primary threats are habitat loss from encroaching tea plantations and illegal gem mining. Climate change presents another danger, as warming temperatures could alter the humid conditions of the cloud forests that these specialized frogs depend on. Researchers regularly discover new species in the sanctuary—eight were described in a single 2013 paper—and many are immediately classified as Critically Endangered due to their small, threatened habitats. The sanctuary has also yielded rediscoveries of species once thought to be extinct, such as Pseudophilautus stellatus, which had not been seen for 160 years.