A brand new study reveals the explanations for the numerous differences between animal species living on opposite sides of certain natural boundaries. Islands, especially isolated ones, have long been a subject of interest in evolutionary biology. Geographic isolation allows species to evolve independently, leading to extraordinary biodiversity.
These boundaries, although invisible, have a profound effect on the distribution and adaptation of animals. For example, within 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 some species.
Similar discoveries were made by Alfred Russel Wallace within the Malay Archipelago, a region consisting of 1000’s of islands characterised by extraordinary biodiversity.
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Wallace, although often in comparison with Darwin, is credited with pioneering our understanding of species distribution. The Wallace Line acts as a barrier to species moving east from Asia, leading to striking differences between species on either side of the road. For example, the Komodo dragon is now confined to islands in eastern Indonesia, although its fossils first appeared in Australia greater than 3 million years ago.
The existence of this line reflects the numerous influence of the ever-changing dynamics of the Earth’s surface, where major tectonic plates have moved and collided for hundreds of thousands of years. The Malay Archipelago, a fancy tectonic area, lies on the junction of several plates.
Historically, through the Ice Age, when sea levels were lower, there have been two landmasses—Palaeo-Sunda within the west and Sahul within the east. The Sahul continent, which incorporates Australia, Tasmania, New Guinea, and the Aru Islands, approached the Sunda continental shelf about 20 to 25 million years ago, through the late Oligocene or early Miocene. The northward shift of the Australian plate brought with it distinct communities of birds, reptiles, and marsupials, further complicating species distribution within the region.
The significance of Wallace’s line lies within the distinct differences between species on either side of it. On the western side, closer to Asia, there are only Asian animals, while on the eastern side, closer to Australasia, there’s a combination of Australasian and Asian species. Wallace also noted that while Asian animals can cross the road, Australasian animals cannot.
The latest study was published on July 6 within the journal Scienceused computer models to simulate the results of climate change attributable to continental shifts on species. The model analyzed greater than 20,000 species on either side of the Wallace Line, taking into consideration aspects similar to dispersal, ecological preferences, and evolutionary relationships.
The results showed that Asian species were higher adapted to the Malay archipelago, while species from the drier Australian continent struggled to survive on the humid tropical islands. On the opposite hand, species that were more tolerant of rainfall were more successful in migrating to Australia.
Why have Asian species adapted higher than Australian species?
Major climate changes prior to now were attributable to the movement of the continents and their impact on Earth’s oceans, not the movement itself. Alex Skeels, lead creator of the study, explains that as Australia moved away from Antarctica, it opened up an area of deep ocean that’s now occupied by the Antarctic Circumpolar Current (ACC), resulting in a cooler global climate.
This extreme climate change affected species on either side of the Wallace Line in alternative ways. Southeast Asia and the Malay Archipelago remained warm and humid, allowing Asian species to adapt more effectively and use the islands as “bridges” to achieve Australia. In contrast, Australian species, which evolved in a cooler, drier climate, had difficulty adapting to tropical islands.
