A new study reveals the reasons for significant differences between animal species living on opposite sides of certain natural boundaries. Islands, especially isolated ones, have long been a focus of evolutionary biology. Geographic isolation allows species to evolve independently, resulting in remarkable biodiversity.
These boundaries, though invisible, have a profound effect on the distribution and adaptation of animals. For example, in the waters off the Cape of Good Hope, the meeting of the Atlantic and Indian Oceans creates an underwater transition zone that serves as a barrier for certain species.
Similar findings were made by Alfred Russel Wallace in the Malay Archipelago, a region of thousands of islands of incredible biodiversity.
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Impact of Plate Tectonics on Species Diversity
Wallace, though often compared to Darwin, is recognized as a pioneer in understanding species distribution. The Wallace Line acts as a barrier to species moving eastward from Asia, resulting in striking differences between species on either side of the line. For example, the Komodo dragon is now found on islands in eastern Indonesia, even though its fossils first appeared in Australia over 3 million years ago.
The existence of this line reflects the significant impact of the Earth's ever-changing surface dynamics, where major tectonic plates have moved and collided over millions of years. The Malay Archipelago, a complex tectonic area, lies at the convergence of several plates.
Historically, two landmasses - Paleo-Sunda to the west and Sahul to the east - existed during the Ice Age when sea levels were lower. The Sahul continent, which includes Australia, Tasmania, New Guinea, and the Aru Islands, approached the Sunda continental shelf about 20 to 25 million years ago, in the late Oligocene or early Miocene. The northward movement of the Australian Plate brought with it distinct communities of birds, reptiles, and marsupials, further complicating the distribution of species in the region.
Wallace Line's Contributions to Understanding Species Distribution
The significance of the Wallace line lies in the stark differences between species on either side of it. On the western side, closer to Asia, only Asian animals are found, while on the eastern side, closer to Australasia, there is a mix of Australasian and Asian species. Wallace also observed that while Asian animals could cross this line, Australasian animals could not.
A recent study, published July 6 in the journal Science, used computer models to simulate the effects of climate change caused by continental shifts on species. The model analyzed more than 20,000 species from both sides of the Wallace Line, taking into account factors such as dispersal, ecological preferences, and evolutionary relationships.
The results showed that Asian species were better suited to the Malay Archipelago at the time, while species from the drier Australian continent struggled to survive on the wet tropical islands. Conversely, species more tolerant of rainfall were more successful in migrating to Australia.
Why Asian species adapted better than Australian species?
Major climate changes in the past have been driven by the movement of continents and their effect on the Earth's oceans, rather than by the movement itself. Alex Skeels, the lead author of the study, explains that as Australia drifted away from Antarctica, it opened up an area of deep ocean now occupied by the Antarctic Circumpolar Current (ACC), leading to a cooler global climate.
This extreme climate shift affected species on either side of the Wallace Line in different ways. Southeast Asia and the Malay Archipelago remained warm and humid, allowing Asian species to adapt more successfully and use the islands as "bridges" to reach Australia. In contrast, Australian species, which evolved in cooler and drier climates, struggled to adapt to the tropical islands.
