Wallace and Opossums

I just finished my term papers for my biology and history classes, so now seems like a good time to describe what I did in my term papers. Especially the biology one, since any Darwin people reading this blog will find it particularly relevant. :-)

For my biology class (the Evolution and Ecology of the South American Biota), I wrote about Alfred Russel Wallace's riverine barriers hypothesis and how it relates to mouse opossums.

(For those who don't know, Wallace was Darwin's co-discoverer of the theory of evolution by natural selection. Darwin rushed into publishing The Origin of Species when he found out that, although he had been accumulating data on evolution for 20 years, Wallace had recently come to the same theory and might well take all the credit if he didn't act soon. We have Wallace to thank for the fact that Darwin published a 400 page book instead of the three volume master work he wanted to write. Wallace also had a gift for noticing geographic relationships between related species and hypothesized migrations based on the relationships. He is known as the father of biogeography.)

This is a photo of Wallace. Source

Wallace wrote a paper in 1852, "On the monkeys of the Amazon" in which he observed the following:

“The Amazon, the Rio Negro and the Madeira [rivers] formed the limits beyond which certain species never passed. The native hunters are perfectly acquainted with this fact, and always cross over the river when they want to procure particular animals, which are found even on the river's bank on one side, but never by any chance on the other.”

This is a map showing the distinct regions of the Amazon Basin that Wallace suggested.
The map comes from: Patton, J.L. & Costa, L.P. (2003) Molecular phylogeography and species limits in rainforest didelphid marsupials of South America. Predators with Pouches: The Biology of Carnivorous Marsupials (ed. by M. Jones, C. Dickman, and M. Archer), pp. 63–81. CSIRO Publishing, Australia.

He focused on primates in his paper, but a lot of recent research on the Amazon River as a biogeographical barrier has focused on mouse opossums. Mouse opossums are a group of marsupials within the opossum family.

This is a photo of a mouse opossum. They are nocturnal, live in trees, and have prehensile (grasping) tails. Source

A lot of the recent investigations using mouse opossums use DNA data in addition to physical comparisons between specimens (what Wallace did). Therefore, modern results can be much more definitive.

So my question was, with mouse opossums as the studied group, to what extent was Wallace correct that the rivers in the Amazon basin form barriers between species?

Some interesting data I found:

Steiner & Catzeflis (2004): 5 species from the Guiana region (north of the Amazon River) have distinct lineages from species south of the river. This means that the river has been a barrier for these species.

Patton & Costa (2003): In one species, Marmosa murina, there are four distinct genetic lineages. The division between two of them is the Amazon River.

Costa (2003): Mouse opossums in two southern genetic lineages are more closely related to each other than to two northern lineages, even though the only separation between one northern lineage and one southern lineage was the river.

These results indicated to me that there is a definite genetic distinction between the north and south sides of the Amazon River. Therefore, Wallace was correct, even though he relied on anecdotes for data. Also, many of the papers I cited had Wallace's 1852 paper in their bibliographies, which I thought was extremely cool since the majority of sources these papers cited were no more than 10 or 20 years old. All in all, writing this paper was a very interesting experience for me. I loved finding Wallace's original paper and comparing the style to modern papers. (Wallace's paper is available here. Unfortunately I cannot link directly to it.) I also love the fact that the work of a great scientist can still be relevant and insightful 150 years later. Wallace is truly amazing. :-)

Here are the citations for the papers I referenced:
Costa, L.P. (2003) The historical bridge between the Amazon and the Atlantic Forest of Brazil: a study of molecular phylogeography with small mammals. Journal of Biogeography, 30, 71–86.

Patton, J.L. & Costa, L.P. (2003) Molecular phylogeography and species limits in rainforest didelphid marsupials of South America. Predators with Pouches: The Biology of Carnivorous Marsupials (ed. by M. Jones, C. Dickman, and M. Archer), pp. 63–81. CSIRO Publishing, Australia.

Steiner, C. & Catzeflis, F.M. (2004) Genetic variation and geographical structure of five mouse-sized opossums (Marsupiala, Didelphidae) throughout the Guiana Region. Journal of Biogeography 31 (6), 959–973.