The Galapagos Islands have become the hallmark of evolutionary theory and the origin of species. Especially the work on Darwin’s Finches by Peter and Rosemary Grant has shed light on the role of natural and sexual selection in the process of speciation. Their work is beautifully summarized by Jonathan Weiner in “The Beak of the Finch“. If you haven’t read it, please do!
You might have the idea that the emergence of a new species is a one-way street that proceeds in distinct stages. Two populations get separated (geographically or ecologically), gene flow between them diminishes or completely stops, and when they come into secondary contact, they are unable to produce viable offspring. And voila, you have two new species! But like most things in biology, speciation is rather messy. In one extreme case, two species might even merge back into one due to hybridization.
This process, called reverse speciation, mostly occurs among adaptive radiations, where the reproductive isolation between the incipient species can be very unstable. A small change in the environment might lead to the collapse of the two species into one. Classic examples are sticklebacks and cichlids [see Seehausen (2006) for more details]. Recently, the Galapagos Islands have added a few examples to the list. Let’s have look at two cases.
The Wolf Volcano, situated on Isabela Island, is the highest peak in the Galapagos. And home to an endemic giant tortoise species, Chelonoidis becki. To the untrained eye, there is only one tortoise species on this island. To the trained eye, however, there is also one tortoise species… But when you look past morphology and zoom in on the genetics, you will find two subpopulations that correspond to the northern and the western slope of the volcano. Previous studies have indicated that Santiago Island tortoises (C. darwini) are the most likely source of the Wolf Volcano colonists. The question now arises whether there was one colonization event followed by speciation or two colonization events that gave rise to these cryptic subpopulations. Genetic analyses by Ryan Garrick (University of Mississippi) and his colleagues revealed two separate colonization events. But they also found that these lineages are fusing by interbreeding. Reverse speciation caught in the act!
Another island, Floreana, houses three species of tree finch: the small (Camarhynchus parvulus), medium (C. pauper) and large (C. psittacula) tree finch. Sonia Kleindorfer from Flinders University in Australia and her colleagues compared the morphology of these three species across three time periods: 1852-1906, 2005 and 2010. The historical data (from the famous ornithologist David Lack, measured in 1947) showed three clear clusters. Surprisingly, in 2005, they could only uncover two clusters. And in 2010 the gap between the 2005-clusters was filled with “intermediate” individuals. It seems that the three tree finches have collapsed into two species from the 1900s until now. Moreover, large-bodied individuals were rare or absent in the most recent samples. So, the large tree finch may be locally extinct on Floreana Island.
Oh, the Irony!
The fact that an island archipelago that became a model for speciation, is now facing several cases of speciation reversal is quite ironic to me. This is part of the attractiveness of (evolutionary) biology; you find unexpected things in unexpected places. But in the end it all makes perfect sense!
Garrick, R., Benavides, E., Russello, M., Hyseni, C., Edwards, D., Gibbs, J., Tapia, W., Ciofi, C., & Caccone, A. (2014). Lineage fusion in Galápagos giant tortoises Molecular Ecology, 23 (21), 5276-5290 DOI: 10.1111/mec.12919
Kleindorfer, S., O’Connor, J., Dudaniec, R., Myers, S., Robertson, J., & Sulloway, F. (2014). Species Collapse via Hybridization in Darwin’s Tree Finches The American Naturalist, 183 (3), 325-341 DOI: 10.1086/674899