River Barrier Hypothesis

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Amazon river system

The River Barrier Hypothesis (also: Riverine Hypothesis , dt .: thesis that rivers form species barriers; river barrier hypothesis) is a scientific hypothesis that should explain the high biodiversity in the Amazon basin . It was first introduced by Alfred Russel Wallace in his 1852 study On Monkeys of the Amazon . The theory is that the course and current of the Amazon and some of its tributaries are large enough that they act as obstacles and thus induce allopatric speciation in some animal populations . Due to different selection pressure and gene drift as well as a lack of gene exchange, the populations on both sides develop apart and the species splits up.

Several clues should be observed if speciation was caused by a river barrier:

  • The species differences on both sides of the river should increase, with increasing width of the river is also very little or not at all in the headwaters be detectable.
  • Organisms that live exclusively on or above solid ground ( terra firme ) should be more affected than those that occur in the alluvial forests (floodplain areas) along the river, as the first have to cover a longer distance before they reach a suitable habitat again and separate lowland populations can be relatively common reunited on the other hand, once a river in the early stages of Altwasserbildung narrowed (oxbow lake) or relocated its course.
  • If a river as a barrier favors the division of species, sister species should appear on opposite banks more frequently than one would expect if they were randomly distributed.

Mechanisms

If a speciation by a flow barrier, if a flow is wide enough to allopatric speciation (speciation by spatial separation, with vikarianter trigger proliferation of the daughter species), or when the flow is large enough to genetic exchange ( gene flow ) between two populations on the to prevent opposite banks. According to the hypothesis, a genetic and morphological divergence of the population of a species, whose individuals cannot cross it, is initiated when the river shifts its course secondarily into its habitat , or when the species moves on both sides of the river from the source to the mouth spreads out, dividing their population on the lower reaches. By chance (floating on floating debris, for example), a small founder population of a species that was initially only native to one side could be transported across the river, which prevents normal expansion.

Usually the probability of a river becoming a barrier is proportional to its width; the greater the distance that has to be overcome, the greater the difficulty that gene exchange continues to take place. The strength of the barrier effect changes in the course of a river; the narrow source streams are easier to cross than the wide river beds in the lowlands. Rivers that form a barrier for certain species in a region are not an obstacle for other species. The migration barrier therefore differs from species to species (or more generally clade to clade) and separates them to different degrees. Isolation and differentiation on the opposite banks show a specific pattern.

Large mammals and birds have little difficulty traversing the large rivers, while small birds, which can only travel short distances in flight, and small mammals are less likely to travel long distances and should be more likely to see an impact. It is also known that numerous bird species avoid flying over open water. By forming meanders, rivers can multiply the distance to be overcome for animals who prefer drier forests. Conversely, the constriction of meander loops as oxbow lakes in floodplains can move land areas inside the loop, with their colonists, from one bank of the river to the other.

Supporting evidence

Many research projects in the Amazon basin aimed to test the validity of this hypothesis. The southern redtail ant bird ( Sciaphylax hemimelaena ) is a species that confirms the hypothesis in nature. The diversification and distribution of the ant bird has been studied in the Amazon. Three monophyletic , genetically delimitable populations could be identified; two of them are now recognized as a subspecies. Two of the clades occur on both sides of the Rio Madeira and the third had its habitat between the Rio Madeira and two tributaries, the Jiparaná and the Rio Aripuanã . The research indicates that this species of bird has split through river barriers by interrupting the flow of genes.

The Mittermeier nude face kaki ( P. mittermeieri ) occurs south of the Amazon between Rio Madeira and Rio Tapajós .

Another study of saddleback tamarins ( Leontocebus sp.) Examined the prediction that gene flow would be different in different sections of the river. It was actually found that the gene flow mainly takes place via the narrower source rivers, while it continues to decrease towards the mouth. This also agrees with the hypothesis. However, it is questionable whether a single mechanism is sufficient to explain the diversification in the tropics. For example, there is evidence that genetic variation in the blue-headed pipra ( Lepidothrix coronata ) can only be explained by a combination of river barriers , Andean folding and spreading of the species.

Rivers separate the distribution areas of the four gibbon species on Borneo. Dotted, the hybridization zone not separated by broad rivers.

In primatology , the river barrier hypothesis is widely accepted. In a revision of the monkey genus , the rainforest expert and primatologist Laura K. Marsh divided the Sakis ( Pithecia ) into 16 species and assumed in her work that the great Amazonian rivers would not be crossed by the monkeys according to the river barrier hypothesis can and thus act as barriers for allopatric speciation. Similar distribution patterns can be seen in the squirrel monkeys ( Saimiri ), e.g. E.g. in the eastern Amazon basin, where the common squirrel monkey occurs north of the Amazon and the very similar Collins squirrel monkey lives south of the Amazon, among the uncrowned capuchins ( Cebus ), the Bartsakis ( Chiropotes ), the Amazonian Saguinus species and among the four species of gibbon ( Hylobates sp.) On Borneo .

criticism

Red-backed tree climber ( Dendrobates reticulatus )

Not all studies found positive evidence for the hypothesis. One study tested the Riverine Hypothesis using populations of four species of frogs along the Rio Juará . The team expected less gene flow when comparing populations on opposite banks to those on the same bank. But this was not the case. Gene flow appeared to have occurred to the same extent in both cases.

Another study tested the hypothesis in a larger context. The researchers assumed that rivers not only represent barriers to gene flow within certain species, but also at the level of entire species communities. For this purpose, the variation in frog species and small mammals along rivers and on the opposite banks of the Juará River was examined. No visible discrepancy was found between the frog species and mammal species at the source rivers and at the river mouth. And no greater similarities were found between the species that live on the same bank of the river than between species that occur on opposite bank sides.

These results partly call the hypothesis into question. The validity of the hypothesis was then further tested by studies on tree diver frogs . It emerged that the populations on the same side of the river were by no means all monophyletically related. In Lougheed's study, it should be shown that the Ridge Hypothesis (mountain ridge barrier theory) is much more likely than the examined hypothesis.

81 species of flightless mammals were trapped in another experiment on opposite banks of the Rio Juará. The river appeared to be a barrier for only a few species, while the majority of the species were either homogeneous in the study area or divided into monophyletic clades between the upper and lower reaches. Patton argues that the geographical distribution of these clades rather suggests that landform evolution (i.e. division according to elevation, subsoil, etc.) is an underestimated factor of diversification in the Amazon.

These studies suggest that other factors also influence species development in the Amazon. Another deficiency of the hypothesis is that it was examined almost exclusively in the Amazon and hardly in other river basins . In addition, it is very likely that regular shifts in the course of the river will prevent the formation of recognizable patterns and, for this reason, it is very difficult to test the credibility of the hypothesis with scientific research.

Individual evidence

  1. ^ AR Wallace: On the monkeys of the Amazon. In: Journal of Natural History , 1854, 14 (84): 451-454.
  2. ↑ cf. on this (examined on the Mississippi in North America): Nathan D. Jackson, Christopher C. Austin (2013): Testing the Role of Meander Cutoff in Promoting Gene Flow across a Riverine Barrier in Ground Skinks (Scincella lateralis). PLoS ONE 8 (5): e62812. doi: 10.1371 / journal.pone.0062812 .
  3. ↑ Alexandre M. Fernandes, Michael Wink, Alexandre Aleixo: Phylogeography of the chestnut-tailed antbird (Myrmeciza hemimelaena) clarifies the role of rivers in Amazonian biogeography. In: Journal of Biogeography. 39 (2012): 1524-535.
  4. ↑ CA Peres, JL Patton, Maria Nazareth F. Da Silva: Riverine barriers and gene flow in Amazonian saddle-back tamarins. In: Folia Primatologica. 67.3 (1996): 113-24.
  5. ↑ ZA Cheviron, SJ Hackett, AP Capparella: Complex evolutionary history of a Neotropical lowland forest bird (Lepidothrix coronata) and its implications for historical hypotheses of the origin of Neotropical avian diversity. In: Molecular Phylogenetics and Evolution. 36.2 (2005): 338-357.
  6. ↑ Laura K. Marsh (2014): A Taxonomic Revision of the Saki Monkeys, Pithecia Desmarest, 1804. Neotropical Primates. 21 (1); 1-163.
  7. ↑ MP Mercês, Jessica W. Lynch Alfaro, WAS Ferreira, ML Harada, José de Sousa e Silva Júnior: Morphology and mitochondrial phylogenetics reveal that the Amazon River separates two eastern squirrel monkey species: Saimiri sciureus and S. collinsi. Molecular Phylogenetics and Evolution, 82, Part B, pp. 426-435, January 2015 (online from October 20, 2014) doi: 10.1016 / j.ympev.2014.09.020
  8. ↑ Claude Gascon, Stephen C. Lougheed, James P. Bogart: Patterns of genetic population differentiation in four species of Amazonian frogs: a test of the riverine barrier hypothesis. In: Biotropica 30.1 (1998): 104-19.
  9. ↑ Claude Gascon, Jay R. Malcolm, James L. Patton, James P. Bogart: Riverine barriers and the geographic distributions of Amazonian species. In: Proceedings of the National Academy of Sciences . 97.25 (2000): 13672-3677.
  10. ↑ Stephen C. Lougheed, Claude Gascon, DA Jones, James P. Bogart, Peter T. Boag: Ridges and rivers: a test of competing hypotheses of Amazonian diversification using a dart-poison frog (Epipedobates femoralis). In: Proceedings of the Royal Society. 266.1421 (1999): 1829-1835.
  11. ↑ Stephen C. Lougheed, Claude Gascon, DA Jones, James P. Bogart, Peter T. Boag: Ridges and rivers: a test of competing hypotheses of Amazonian diversification using a dart-poison frog (Epipedobates femoralis). In: Proceedings of the Royal Society. 266.1421 (1999): 1829-1835.
  12. ↑ James L. Patton, Maria Nazareth F. Da Silva, Jay R. Malcolm: Mammals of the Rio Juruá and the evolutionary and ecological diversification of Amazonia. In: Bulletin of the American Museum of Natural History 244 (2000) : 1-306

literature

  • G. Voelker, BD Marks, C. Kahindo, U. A'genonga, F. Bapeamoni, LE Duffie, JW Huntley, E. Mulotwa, SA Rosenbaum, JE Light: River barriers and cryptic biodiversity in an evolutionary museum. In: Ecology and evolution. Volume 3, number 3, March 2013, pp. 536-545, doi : 10.1002 / ece3.482 , PMID 23532272 , PMC 3605844 (free full text).
  • RK Colwell: A barrier runs through it ... or maybe just a river. In: Proceedings of the National Academy of Sciences. 97, 2000, p. 13470, doi : 10.1073 / pnas.250497697 .