Did Past Climate Changes Promote Speciation in the Amazon?
Any time you've got a whopping big river like the Amazon (or a mountain chain like the Andes, or an ocean, or whatever), you've gotta figure that it will be a biogeographical barrier. Depending on the kind of organisms, big rivers, high mountains, oceans, forests, deserts, and so on can provide a habitat or a barrier, and when there is a barrier, populations may end up splitting across that barrier and diverging to become novel species.
The role of the big tropical rivers such as the Amazon and the Congo, and the role of rain forests, in certain actual speciation events is pretty clear, but the contribution of these potential barriers to overal species diversity and the general pattern of phylogeny in the wetter tropics is not well understood. This is because the kind of data needed to measure the effects of these potential barriers is hard to come by in these habitats.
A new study in PLoS looks at the role of past climate changes and sea level fluctuations in the rise of new species in the Amazon region.
Overview of populations sampled and groupings used in hypothesis tests (left to right: Atta cephalotes, Atta sexdens, Atta laevigata).
A-C: Results of maxent binary distribution models for the three species under current conditions. Areas predicted to be suitable for each species under current climate conditions are shaded in green. Populations used in this study are shown with red circles; populations for which molecular data were obtained are indicated by filled circles, while populations used only for distribution modeling are indicated by open circles. D-F: Paleodistributions of the three species at the LGM (21 kya) estimated by projecting the maxent model for current conditions onto climate layers from the LGM. Red circles indicate populations used in molecular analyses; Regions outlined with colored lines show population groupings used to test the Pleistocene refugia hypothesis. G-I: Population groupings used to test the marine incursion hypothesis are circled with colored lines (red = Andes, blue = Guiana Shield, yellow = Brazilian Shield); populations for which molecular data were obtained are indicated by filled circles. J-L: Populations used to test the riverine barrier hypothesis are shown with yellow or red circles, indicating populations located north or south of the Amazon river, respectively. Populations for which molecular data were obtained but are located away from the Amazon river (and are therefore not considered in tests of this hypothesis) are shown with empty, black circles.
The huge Amazon basin harbors the richest diversity of life on earth, but you have to start somewhere, so this team of American and Brazilian researchers picked three species of leafcutter ants from Central and South America to unravel the mystery of the role geography and climate may have played in the ants' phylogenetic history.
According to Scott Solomon, lead author, "One way in which our study is unique is that we looked at an insect. Previous studies have focused mostly on birds, mammals and other vertebrates, whereas insects actually represent the majority of the animal diversity in the Amazon."
Historically, the key determinants of speciation through biogeography in the Amazon were thought to be the big rivers as barriers, and the breaking up of the rain forest into refugia during glacial maxima over the last 1.5 million years or so.
From the abstract:
...The leading hypotheses on allopatric diversification of Amazonian taxa are the Pleistocene refugia, marine incursion, and riverine barrier hypotheses. Recent advances in the fields of phylogeography and species-distribution modeling permit a modern re-evaluation of these hypotheses. Our approach combines comparative, molecular phylogeographic analyses using mitochondrial DNA sequence data with paleodistribution modeling of species ranges at the last glacial maximum (LGM) to test these hypotheses for three co-distributed species of leafcutter ants (Atta spp.). The cumulative results of all tests reject every prediction of the riverine barrier hypothesis, but are unable to reject several predictions of the Pleistocene refugia and marine incursion hypotheses. Coalescent dating analyses suggest that population structure formed recently (Pleistocene-Pliocene), but are unable to reject the possibility that Miocene events may be responsible for structuring populations in two of the three species examined. The available data therefore suggest that either marine incursions in the Miocene or climate changes during the Pleistocene--or both--have shaped the population structure of the three species examined. Our results also reconceptualize the traditional Pleistocene refugia hypothesis, and offer a novel framework for future research into the area.
In case you were wondering, marine incursions in the Miocene was not a matter of the Amazonian estuary growing a few hundred miles. During much of the Miocene what is now the Amazon basin was part of a large inland basin that communicated with the then open ocean north of South America (there was no Central America) or the Pacific, NOT the Atlantic as it does today. This helps explain some interesting mystery like how some of the fresh water organisms living in the Amazon that evolved from ocean organisms are more closely related to Pacific than to Atlantic forms.
Solomon, S.E., Bacci, M., Martins, J., Vinha, G.G., Mueller, U.G., Bennett, P.M. (2008). Paleodistributions and Comparative Molecular Phylogeography of Leafcutter Ants (Atta spp.) Provide New Insight into the Origins of Amazonian Diversity. PLoS ONE, 3(7), e2738. DOI: 10.1371/journal.pone.0002738