A forest divided by ice
The Wet Tropics of Queensland is not a continuous, ancient forest. It is a mosaic of habitats, formed by millions of years of climate change. During the Pleistocene epoch, which began about 2.5 million years ago, a series of ice ages dramatically altered this landscape. As glaciers expanded in the planet's polar regions, the climate in northern Australia became cooler and much drier. The vast, humid rainforest contracted, surviving only in wetter, mountainous areas that were isolated refugia.
One of the most significant barriers created by this drying was the Black Mountain Corridor, a drier, rockier gap that separated the rainforests to the north (Carbine and Thornton Uplands) from those to the south (Atherton Uplands). For thousands of years, populations of rainforest-dependent animals were completely separated. Trapped in their respective refugia, these isolated groups began to evolve independently. They accumulated distinct genetic mutations, slowly drifting apart from their relatives on the other side of the arid gap. When the climate warmed and became wetter at the end of the last glacial period, the rainforest expanded and the two sections were reconnected. Where these two long-separated populations now meet is known as the Wet Tropics Suture Zone.
A living evolutionary laboratory
The suture zone shows evolution. Here, scientists can study what happens when genetically distinct lineages come back into contact after long periods of isolation. Studies have identified numerous species that show deep genetic divisions across this zone. Many of these are "cryptic species"—animals that look identical to one another but are genetically very different.
The Northern Leaf-tailed Gecko (Saltuarius cornutus) is an example. Populations north and south of the corridor show significant genetic divergence. Another well-studied resident is the Prickly Forest Skink (Gnypetoscincus queenslandiae), a creature that lives its entire life within rotting logs on the forest floor and is rarely found outside the rainforest. Its limited movement makes it an excellent subject for tracking genetic lineages. Studies on various skinks, geckos, and birds have revealed mitochondrial DNA sequence divergences ranging from 2% to 15% between northern and southern populations.
In some cases, the separated populations can still interbreed where they meet, creating hybrid zones. In other cases, they have diverged so much that they are now reproductively isolated, effectively becoming new species. This makes the suture zone a rare opportunity to observe the process of speciation—the formation of new and distinct species in the course of evolution—in action. The interactions between these lineages vary widely, from complete mixing to the formation of distinct species, showing evolutionary processes.