Indonesia's Wallace Line and an Introduction to Biogeography

Poster from Science-art.com

"In the archipelago there are two distinct faunas rigidly circumscribed, which differ as much as those in South America and Africa and more than those of Europe and North America.  Yet there is nothing on the map or on the face of the islands to mark their limits.  The boundary line often passes between islands closer than others in the same group  --Alfred Russel Wallace, 1858 in a letter to his friend Henry Walter Bates.


Alfred Russel Wallace is often described as a "naturalist".  He lived from 1823 to 1913, a time when many of the principles guiding our understanding of the world and its natural processes were being formulated.  Wallace went on expeditions around the world; he suffered malaria and dysentery on his quest for knowledge and several times nearly lost his life.  But his real contributions would stem from his time in the islands that would eventually become known as Indonesia.

Alfred Russel Wallace explored the Malay Archipelago from April 1854 to April 1862.  During that time he collected thousands upon thousands of specimens and described hundreds of new species.  He was fascinated by the diversity of species in Indonesia.  He was startled to find dozens of species of trees, butterflies, and other creatures within close proximity to one another.  The diversity of species was far beyond that in his native England, and what he found challenged conventional understandings of where creatures come from.  Among these diverse creatures, though, Wallace found similarities, and reasoned that many of the species he encountered must be somehow related.  What complicated his hypotheses, though, was the existence of a stark contrast in species composition between the eastern and western parts of the archipelago.  In the words of Van Oosterzee (see reference below),

In Borneo Wallace found monkeys, wild cats, civets, otters, squirrels, but in Sulawesi he found few monkeys, but lots of cuscuses.  In Sulawesi there were honeyeaters and parrots, but in Borneo there were oriental birds like woodpeckers, barbets, trogons, fruit thrushes...

The two sides are divided by what would eventually become known as the Wallace Line (1).  Wallace had to figure out what was going on here.  What was responsible for these incredible variations in floral and fauna between islands that in some cases were only a few kilometers apart?

Wallace is often considered the father of a burgeoning sub-discipline of geography called biogeography.   Wallace was the first scientist to realize that the distribution of related species is fundamentally linked to the geologic history of the places the species inhabit.  Biogeographers study physical geography in an attempt to understand how species are distributed.  Biogeography is based on the principles of evolution and natural selection that were first elucidated by Charles Darwin and the aforementioned Alfred Russell Wallace. A proper discussion of the theory of evolution is beyond the scope of this blog, but for our purposes here a couple of basic principles will suffice:

1.  There is random, natural variation within species.  This simply means that through time, a given species will change bit by bit.  You can see this in humans.  Even though as a species we are very closely related to one another, there are all sorts of differences between people.  We have different skin tones, eye color, hair color, body types, etc.  All of these differences are types of mutations that occurred during the several-million-year evolutionary history of humans!  The same thing happens with other species as well.  Think about all the different types of dog you see.  They are all the same species (canis familiaris), but between the different breeds there is a lot of variation. 

2.  This random variation can be passed down through generations.  Even though the variation is random, it can be passed down from generation to generation because the information responsible for these variations is encoded in genes.  This is why you share many of the same traits as your parents!  In the aforementioned example of dogs, people take advantage of this tendency for traits to be inherited to breed dogs of certain types. 

3.  New traits that confer an advantage will be more likely to be handed down, whereas traits that are disadvantageous are less likely to be passed along.  I like to think of these variations as nature's laboratory.  Right now nature is performing millions upon millions of experiments on living things!  Sometimes the new trait might help a critter live longer or cope with its environment better.  In this case, the critter is more likely to find a mate and have children.  But in some cases the new trait might cause problems for the critter, which would make it more likely to be eaten by a predator or befall some other unfortunate case.  If this happens, it's less likely that the new trait will be passed along. 

Photo from Hawaiian Tree Snail Conservation Lab at UH

The unique contribution of Wallace was to show that the lay of the land matters in these processes of evolution and speciation.  So in addition to the aforementioned basic principles we can add a couple of correlaries:

1.  Species that are somehow divided into subpopulations will begin to diverge, and the rate of divergence depends in part on how frequently the populations interact with one another. 

Like Indonesia, Hawai'i is a great place to study biogeography.  One fascinating example of the effect of the natural environment on the evolution and radiation of species comes from the many varieties of tree snails that inhabit our archipelago.  When the snails arrived in Hawai'i long ago, there were no predators, and so the random, natural variation we spoke of earlier ran rampant.  The snails were able to spread all over the island.

This is where changes in the landscape come in.  Have a look at the two topographical maps I've included below.  The first is of the Big Island, which is the youngest island in the archipelago.  It's actually still growing.  The second map is of Oahu.  Can you notice any differences?


Even though these maps are of slightly different types, you should be able to see some differences between the two islands.  Oahu is marked by valleys running out of the Koolau and Wainae mountain ranges.  Manoa, Makua and Palolo are just two examples; there are dozens of others, and you're probably familiar with at least a few of these.  These valleys are carved by wind and water, the forces of erosion.  The oldest part of Oahu is a little more than 3 million years old, and so mother nature has had a long time to work her magic on the island.  The island has been dissected by water and wind, leaving a roughness to the terrain.  The Big Island, on the other hand, doesn't have nearly as many valleys (though there are a few), and though the mountains are higher, the terrain is a lot smoother. 

Now let's think about the islands from a snail's perspective.  Remember our islands have emerged from a hot spot (TK) in the middle of the ocean, and so when they emerge from the sea they are barren with no creatures.  They are also very far from any other land masses (think of how long it takes to get anyplace on a plane!) which could be potential sources for colonizing creatures.  Thus there are significant obstacles for any plant or animal to make it to Hawai'i, and throughout the millions of years that the islands have been here only a few managed to make the journey.  The tree snail was one of the lucky ones.  When the tree snail arrived millions of years ago, it likely disembarked on an island like the Big Island: expansive with fairly smooth terrain.  It would have been easy for the snail to spread across this island.  Combine this with the fact that there were no predators and you have a true snail paradise!  I've created a diagram below to give you an idea of what it might look like (please note that the snails are not to scale):

As you can see, the snails are free to roam and interact.  They are pretty much all a part of the same population, and so the random variation gets shared between them because they are able to breed with one another.  We can think of the Big Island as being like this.  However, as years pass and erosion sets in, natural obstacles like valleys and ridges are worked into the landscape, as you can see from the diagram below.  This diagram is a model for an older island, like Oahu or Kaua'i.

On the older island, the initial population of snails gets broken up into smaller populations, each isolated from one another.  What this means is that if a new trait emerges in one of these smaller populations it will be shared within that population only, but not with the other populations.  So eventually the different populations will begin to diverge and will eventually reach the point where they are completely different species!

Wallace was the first to understand how natural geologic processes could affect the evolution and divergence of species, but he made his observations in the context of Indonesia.  His "line" traces a division rooted tens of millions of years in the past when the configuration of continents was very different than today.  The variation in species reflects the varying origins of the various islands in the Indonesian Archipelago.  The islands of the western part of Indonesia have a very different history from those of the eastern part, and the island of Sulawesi is a different story all together!  Look for a future post on how the islands of Indonesia have actually moved across the face of the earth.

(TK)  If you are not familiar with the way the Hawaiian Islands form, you can do a google search for "Hawaiian Island formation" or "volcanic hot spot".  The Hawaiian Center for Volcanology is a good place to start.  The Bishop Museum in Honolulu had a really good exhibit on the hot spot as well.

References: Van Oosterzee, 1997.  When Worlds Collide: The Wallace Line. Ithaca, NY: Cornell University Press.  220pp.