What Is the Wallace Line?

Darwin's colleague had his own findings

Alfred Russel Wallace Line
Map of the Malay Archipelago by Alfred Russel Wallace.

Flickr / Wellcome Library, London / Wellcome Images

Alfred Russel Wallace may not be well known outside the scientific community, but his contributions to the theory of evolution were invaluable to Charles Darwin. In fact, Wallace and Darwin collaborated on the idea of natural selection and presented their findings jointly to the Linnean Society in London. However, Wallace has become just a footnote in history due to Darwin publishing his book "On the Origin of Species" before Wallace could publish his own work. Even though Darwin's findings used data that Wallace contributed, Wallace still did not get the sort of recognition and glory that his colleague enjoyed.

There are, however, some great contributions Wallace does get credit for from his journeys as a naturalist. Perhaps his best-known finding was discovered with data he gathered on a trip through the Indonesian islands and surrounding areas. By studying the flora and fauna in the area, Wallace was able to come up with a hypothesis that includes something called the Wallace Line.

What Is the Wallace Line?

The Wallace Line is an imaginary boundary that runs between Australia and the Asian islands and the mainland. This boundary marks the point where there is a difference in species on either side of the line. To the west of the line, for instance, all of the species are similar or derived from species that are found on the Asian mainland. To the east of the line, there are many species of Australian descent. Along the line is a mix of the two, where many species are hybrids of the typical Asian species and the more isolated Australian species.

The Wallace Line theory holds true for both plants and animals, but it is much more distinctive for the animal species than the plants.

Understanding the Wallace Line

There was a point in time on the Geologic Time Scale where Asia and Australia were joined together to make one giant landmass. During this period, species were free to move about on both continents and could easily remain one singular species as they mated and produced viable offspring. However, once continental drift and plate tectonics started to pull these lands apart, the large amount of water that separated them drove evolution in different directions for the species, making them unique to either continent after a long period of time had passed. This continued reproductive isolation has made the once closely related species disparate and distinguishable.

Not only does this invisible line mark the different areas of animals and plants, but it also can be seen in the geological landforms in the area. Looking at the shape and size of the continental slope and continental shelf in the area, it seems that the animals observe the line by using these landmarks. Therefore, it is possible to predict which types of species you will find on either side of the continental slope and the continental shelf.

The islands near the Wallace Line are also collectively called by a name to honor Alfred Russel Wallace: Wallacea. They also have a distinctive set of species that live on them. Even the birds, which are capable of migrating between the mainlands of Asia and Australia, seem to stay put and have thus diverged over long periods of time. It is not known if the differing landforms make the animals aware of the boundary, or if there is something else that keeps the species from traveling from one side of the Wallace Line to the other.