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DISPLAYING POSTS TAGGED: wallace line (2)

The Age of Exploration continues

Author
by Chenae Neilson
Publish date
6 November 2013
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Chenae Neilson is a Master of Science in Human Geography student at the University of Melbourne. This is the first post of an MV Blog mini-series celebrating the past, present and future of exploration on planet Earth and commemorating the adventures of Alfred Russel Wallace who died 100 years ago today.

Over the last 250 years, the taxonomic classification of life on Earth has described approximately 1.2 million species that share the planet with us, Homo sapiens. However, every year, we discover thousands of new species. Often these are organisms that have been concealed from scientific observation within impassable forests or the dark of the ocean abyss. A 2011 estimate suggests there are over eight million species on Earth, which means we have described fewer than 15 per cent of them. This stems, in part, because we have explored only five per cent of the world’s oceans, and scientists have yet to survey many terrestrial environments.

This celebration of exploration marks the centenary of the death of explorer, naturalist, and evolutionary biologist Alfred Russel Wallace who died on 7 November 1913. To commemorate Wallace we return to the region of his most famous exploration.

Alfred Russel Wallace, circa 1895. Alfred Russel Wallace, circa 1895.
Image: London Stereoscopic & Photographic Company
Source: Wikimedia Commons

"Nature has taken every precaution that these, her choicest treasures, may not lose value by being too easily obtained."
Alfred Russel Wallace

 

From 1854 to 1862, Wallace traveled across the Indonesian archipelago from the Sunda shelf in the west through the island of Sulawesi and to the Bird's Head of New Guinea in the east. On this journey he noted a distinct transition from more Asian fauna to more Australian fauna – the line demarking this dramatic transition became known as the 'Wallace's Line'. The broader region between Asia and Australia, including the island of Sulawesi, is termed Wallacea, which hosts an endemic combination of Asian and Australian lineages. Wallace’s exploration of Sulawesi made a significant contribution to the species catalogue in this region and also became the backbone of evidence for his own theory of evolution by natural selection. Wallace’s adventures in Wallacea were later chronicled in his book, The Malay Archipelago, written in 1869.

Map of Sunda and Sahul Map of Wallacea, featuring the island of Sulawesi and the biogeographic lines defining breaks between Asian fauna within Sunda and Australasia.
Image: Maximilian Dörrbecker
Source: Used under CC BY-SA 3.0 from Wikimedia Commons.
 

Despite the countless scientific findings made during the Age of Exploration in the 1800s, exploration still remains important today, particularly in a world where natural environments are under threat.

Wallace's trail of exploration has been retraced many times by contemporary scientists, amateur naturalists, and television personalities. Exploration may mean something different to each. For the scientist, these areas are revealing new species and answering questions about the evolutionary history of the region. To the naturalist, the 'discovery' of animals found nowhere else on earth and the chance to walk in the footsteps of an influential figure may be the very definition of exploration. Regardless of our definition of exploration, areas like Wallacea hold many new possibilities.

Over 100 years later, two research scientists at Museum Victoria, Dr Kevin Rowe and Dr Karen Rowe, have been following in the footsteps of Wallace. Some of their recent findings will be the subject of the next blog in our mini-series.

Woman in rainforest in Indonesia Karen Rowe hiking into a remote field camp on Mount Dako, Sulawesi where scientists from MV, Indonesia and the United States inventoried mammals and birds for the first time in 2013.
Image: Kevin Rowe
Source: Museum Victoria

Wonderful Wallacea

Author
by Kevin Rowe
Publish date
12 January 2012
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Kevin is the Senior Curator of Mammals at Museum Victoria. He reports on his recent expedition to the mountains of Sulawesi, Indonesia in this series of blog posts.

Last Sunday was the birthday of Alfred Russel Wallace who, along with Darwin, co-founded the theory of natural selection. As Ursula reported, Wallace's expedition through the Malay Archipelago (modern-day Indonesia) also prompted him to develop the field of biogeography and to define the zoogeographical regions of our planet; that is, where groups of animals are found and why.

World map showing the zoogeographical regions. Wallace's world map showing the zoogeographical regions in different colours.
Image: A.R. Wallace
Source: Copyright expired. Sourced from Wikimedia Commons.
 

Biogeography proposes that the species native to a particular region are determined by geographic processes such as vicariance (isolation) and dispersal (colonisation). Natural selection then drives species to evolve traits suited to survival and reproduction in their environments available in their geographic context. Biogeography and natural selection combined often result in species from different regions converging on similar morphological solutions to similar selective pressures.

Consider the mammals of Australia, which are the product of millions of years of natural selection acting on a set of species determined by biogeography. Australia, along with New Guinea, comprises the continental shelf of Sahul that today, as well as in Wallace's time, is separated by the Indonesian archipelago from the Asian continental shelf, Sunda. During low sea levels of the Pleistocene (the last 2 million years), Australia, New Guinea, and adjacent islands were one continuous landmass as were Malaysia, Sumatra, Java, and Borneo on the Asian side. Between them was Sulawesi, the Moluccan islands and the Lesser Sunda islands (Nusa Tenggara) all separated by deep ocean channels. This region at the interchange between Asia and Australia is referred to as Wallacea.

Map of Sunda and Sahul Map of Sunda and Sahul and the Wallace Line, the Lydekker Line and the Weber Line.
Image: Maximilian Dörrbecker
Source: Used under CC BY-SA 3.0 from Wikimedia Commons.
 

85 million years ago Australia was connected to Antarctica and via Antarctica to South America. When a meteorite crashed into the Yucatan peninsula of North America 65 million years ago annihilating the dinosaurs (except a few feathered and crocodilian ones), Australia was already starting to drift away from Antarctica. For 40 million years Australia drifted in isolation and only about 10 million years ago began to collide with Asia. The mammals that made this journey survive today as the marsupials and monotremes that are unique to Australia and New Guinea. The marsupials have evolved to fill many ecological roles with adaptations similar to placental mammals on other continents, a process driven by natural selection leading to convergent evolution.

Convergent evolution between the Thylacine and the Red Wolf. An example of convergent evolution. The Thylacine (left) and the Red Wolf (right) occupy similar roles in the ecosystem and have similar dog-like characteristics, even though they belong to entirely different groups of animals.
Image: Left: Heath Warwick | Right: ucumari
Source: Left: Museum Victoria | Right: Used under CC BY-NC-ND 2.0 from ucumari.
 

The native terrestrial mammals of Australia, however, are not just the result of isolation but are also descended from a select group of intrepid colonisers from Asia. These are the native bats and rats that account for nearly half of the mammal species of Australia. The rats are remarkable because they are the only non-flying, terrestrial mammals to colonize Australia before humans and they did it twice; once 5 million years ago (Rowe et al, 2008) and again 1 million years ago (Rowe et al, 2011). Both colonisers clearly came from Asian sources but the details of how they crossed multiple ocean channels and archipelagos are still unclear. In part this is hindered by our limited understanding of rat diversity from Wallacea, a condition that I and my colleagues endeavour to correct with our recent expeditions to Sulawesi.

Links:

More on convergent evolution of marsupial and placental mammmals

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