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.
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 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.
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.
More on convergent evolution of marsupial and placental mammmals