What is the Wallace Line?

Alfred Russel Wallace's trip to Indonesia helped him define the Wallace Line
Chief's house on Indonesian island of Sumatra, from Alfred Russel Wallace's (1823-1913) expedition in Australia, engraving, 19th century. Getty/DEA Picture Library

Alfred Russel Wallace may not be well known outside of 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 own findings jointly to the Linnean Society in London. Alfred Russel Wallace has become not much more than a footnote in history in that regard due to Darwin publishing his book On the Origin of Species before Wallace could publish his work.

Even though Darwin's findings were considered ​more complete with the data that Wallace contributed, Alfred Russel Wallace still did not get the sort of recognition and glory that his colleague Charles Darwin enjoyed.

There are, however, still many great contributions Alfred Russel Wallace gets credit for discovering on his journeys as a naturalist. Perhaps his most well-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 a part called the Wallace Line.

The Wallace Line is an imaginary boundary that runs between Australia and the Asian islands and 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, 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 that of Australian descent. Along the line is a mix of the two and many species are hybrids of the typical Asian species and the more isolated Australian species.

At one point in time on the Geologic Time Scale, Asia and Australia were joined together to make one giant land mass.

During this period, species were free to move about on to both continents and could easily stay one 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 ended up separating 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 much different and distinguishable. Even though the Wallace Line theory holds true for both plants and animals, it is much more distinctive for the animal species than the plants.

Not only does this invisible line mark the differing areas of animals and plants, it can also 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. 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.

These islands are known as Wallacea and they also have a very distinctive set of species that live on them. Even the birds, which are capable of migrating to and from the mainlands of Asia and Australia seem to stay put and have diverged over long periods of time. It is not known if the differing landforms serve as a way for the animals to know the boundary, or if it is something else that keeps the species from traveling from one side of the Wallace Line to the other.