A grey mouse lemur (Microcebus murinus). Image from Wikipedia.
Charles Darwin’s visit to the Galapagos Archipelago has been celebrated time and again for its influence on his evolutionary thoughts, but I have to wonder what would have happened if the Beagle skipped the Galapagos and visited Madagascar instead. What would Darwin have made of the animals which had been evolving in splendid isolation on the African island? Would “Darwin’s lemurs”, rather than Darwin’s finches, be among the most recognizable icons of evolution?
Answers to such questions are beyond our grasp, but the diverse array of lemurs which dwell on Madagascar are a stunning example of evolutionary diversification on an island. After the ancestors of modern-day lemurs rafted across the Mozambique Channel between 50 and 60 million years ago, the primates evolved and diversified into a variety of forms, including the 100 or so species endemic to the island today. As suggested by a new study published this week in PLoS One, there may even be more species yet to be found.
Species are commonly distinguished from their close relatives on the basis of physical characteristics, but genetic studies have suggested that there are a number of “cryptic” lemur species which are identifiable by unique genetic signatures in particular populations. This meshes well with what we know about how new species originate. A new lemur species will get its start as a population which is physically identical to, but genetically isolated from, a population of a parent species. This means that the genetic makeup of a diverging lineage will begin to change before physical appearances do, and so studies of lemur genetics has the potential to identify populations that are diverging from their parent populations.
A map marking the locations of mouse lemur populations represented in the study. From the PLoS One paper.
To see if such divergent lineages could be identified among Madagascar lemurs, a team of researchers led by David Weisrock looked at genetic samples from 286 individual mouse lemurs (Microcebus), most of those (216) coming from wild populations which survive in the dwindling forest along the coasts of the island. DNA sequences from both the nucleus of cells and the sausage-shaped organelles called mitochondria were compared, and what the scientists found was that there was more genetic disparity between certain mouse lemur populations than might be apparent just by looking at them. Based upon the mitochondrial DNA, especially, the authors were able to identify populations of the same species which are diverging from each other (such as the populations of the grey mouse lemur [Microcebus murinus] found in Bemanasy and Mandena, respectively), including three diverging lineages that had not been detected before.
While the authors do note that their hypothesis is provisional, the end result of their analysis recovered as many as 16 genetically distinct mouse lemur populations, some of which fit in with known species while others may represent diverging lineages. The question is whether each of the cryptic lineages represents a new species. The authors argue that, since new species become distinguished when they begin to genetically diverge from their parent stock, each of the 16 genetic lineages could be recognized as a distinct species, though other researchers might prefer different criteria based upon reproductive isolation, physical characteristics, or other indicators that show up later after divergence. Regardless of where we might choose to draw the line, though, on the basis of the genetic data is appears that lineages of mouse lemurs are continuing to diverge and evolve in the remaining forested habitat around Madagascar. There are probably additional cryptic lineages that have yet to be found, too, and as scientists continue to sample and compare the DNA of these primates they will help us better understand what Darwin himself spent so much time considering; the origin of species.
Weisrock, D., Rasoloarison, R., Fiorentino, I., Ralison, J., Goodman, S., Kappeler, P., & Yoder, A. (2010). Delimiting Species without Nuclear Monophyly in Madagascar’s Mouse Lemurs PLoS ONE, 5 (3) DOI: 10.1371/journal.pone.0009883