Population genomic structure in six codistributed Amazonian bird species groups. For each species, results of the best k value for STRUCTURE analysis of split datasets (left) and example replicates of the t-distributed stochastic neighbor embedding (t-SNE) analysis (right) are shown. Colored bars underneath the STRUCTURE plots and circles on the t-SNE plots are colored on the basis of major interfluvial regions in map 1 (top left). Map 2 (top right) shows the topography of the study region within the dark red box, where yellow contours represent the 250- to 300-m elevational zone demarcating dynamic lowland (>250 m; shaded in blue) from relatively stable upland (>300 m; shaded in red) basins. Map 3 (top right inset) shows precipitation during the driest annual quarter (76), with high precipitation in gray, low precipitation in red, and a strong cline across the middle to lower reaches of the rivers draining the Brazilian Shield (plotted using QGIS).
The major driver of evolutionary diversity is environmental change because it is the environment the species finds itself in that selects for fitness to survive and reproduce in that environment. This is why species can superficially look as though they are designed to suit their particular environment.
So, it is not surprising that a group of scientists led by Dr. Lukas Musher of the American Museum of Natural History’s Richard Gilder Graduate School found that changes in the local environment in the Amazon River basin are what led to the rich biodiversity in the Amazon Rain Forest. What was surprising, however, was the fact that it was dynamic changes in the courses of the many small tributaries to the Amazon that were the major factor and that proliferating bird species were the result, despite the fact that small rivers do not present major barriers to flying birds.
The team also discovered that this dynamic system had produced several local micro-species along the banks of these rivers, which are vulnerable to extinction.
From the American Museum of Natural History press release:
One of the most contentious questions in evolutionary biology is, how did the Amazon become so rich in species?The team's findings were published, open access, in the journal Science Advances:
A new study focused on birds examines how the movements of rivers in the Amazon havecontributed to that area’s exceptional biological diversity. The research team, led by the American Museum of Natural History, found that as small river systems change over time, they spur the evolution of new species. The findings also reveal previously unknown bird species in the Amazon that are only found in small areas next to these dynamic river systems, putting them at high risk of imminent extinction. The study is detailed today in the journal Science Advances.Early evolutionary biologists like Alfred Russel Wallace noticed that many species of primates and birds differ across opposite riverbanks in the Amazon, and ornithologists now know that rivers are associated—in one way or another—with the origin of many avian species. Moreover, accumulating geological evidence has suggested that these rivers are highly dynamic, moving around the South American landscape over relatively short time periods, on the order of thousands or tens of thousands of years.
Though we know Amazonian biodiversity is unmatched by any other terrestrial ecosystem, we demonstrated that its species richness may be greatly underestimated even in well-studied groups such as birds. Our results corroborate those of other studies that have reported fine-scale patterns of diversity across the southern Amazon basin—a region threatened by rapid and ongoing deforestation—yet this diversity is generally unrecognized. Many of the distinct populations are relatively young and endemic to a small Amazonian region, meaning that a large portion of the Amazon’s birds may be threatened with loss to imminent extinction.Dr. Lukas Musher, lead author
Academy of Natural Sciences
Drexel University, Philadelphis, PA, USA
And Department of Ornithology
American Museum of Natural History, New York, USA.
The lowland rainforests of the Amazon River basin harbor more diversity than any other terrestrial ecosystem on the planet. It is also a globally important biome containing about 18 percent of all trees on Earth and carrying more fresh water than the next seven largest river basins combined. Researchers have long wondered and hotly debate how the Amazon’s rich biodiversity arose and accumulated.
[…]
To investigate how the movement of rivers across the landscape has influenced the accumulation of bird species in the Amazon, the researchers sequenced the genomes of six species of Amazonian birds.
However, because these rivers move around the landscape at different time scales, their movements can have varying outcomes for bird species: when river rearrangements occur quickly, populations of birds on each side can merge before they’ve had time to differentiate; when river changes happen slowly, species have a longer time to diverge from one another; and when rivers change at intermediate rates, bird populations diverge and then join back together and co-occur when a river moves.Even though birds can fly, our study confirmed that current rivers across the Southern Amazon rainforest, even relatively small ones, are highly effective at isolating populations of these six species, which leads to genomic divergence and ultimately speciation.
Joel Cracraft, Senior author
Curator-in-charge
Department of Ornithology
American Museum of Natural History, New York, USA
The scientists also identified distinct populations of birds that should be described as separate species but have been considered a single species until now.
Copyright: © 2022 The authors. Published by American Association for the Advancement of ScienceAbstractIn other words, what the researchers found was that, driven by a dynamically changing environment, the pattern of speciation, partial speciation and remixing is also dynamic. This shows how speciation is not an event but a process where small differences, amplified by environmental selectors, accumulate over time until species diverge to form separate species. But, if physical barriers are removed before the process is complete, diverging species can merge to form a single species again, and occasionally, a new species can arise by hybridization.
Large Amazonian rivers impede dispersal for many species, but lowland river networks frequently rearrange, thereby altering the location and effectiveness of river barriers through time. These rearrangements may promote biotic diversification by facilitating episodic allopatry and secondary contact among populations. We sequenced genome-wide markers to evaluate the histories of divergence and introgression in six Amazonian avian species complexes. We first tested the assumption that rivers are barriers for these taxa and found that even relatively small rivers facilitate divergence. We then tested whether species diverged with gene flow and recovered reticulate histories for all species, including one potential case of hybrid speciation. Our results support the hypothesis that river rearrangements promote speciation and reveal that many rainforest taxa are micro-endemic, unrecognized, and thus threatened with imminent extinction. We propose that Amazonian hyper-diversity originates partly from fine-scale barrier displacement processes—including river dynamics—which allow small populations to differentiate and disperse into secondary contact.
Musher Lukas J.; Giakoumis, Melina; Albert, James; Del-Rio, Glaucia; Rego, Marco; Thom, Gregory; Aleixo, Alexandre; Ribas, Camila C.; Brumfield, Robb T.; Smith, Brian Tilston; Cracraft, Joel (2022)
River network rearrangements promote speciation in lowland Amazonian birds
Science Advances 8(14); doi: 10.1126/sciadv.abn1099
Open access
Reprinted under a Creative Commons Attribution-NonCommercial 4.0 International license (CC BY-NC 4.0)
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