- In the nineteenth century, British naturalist Alfred Russel Wallace, who developed the theory of natural selection independently of Charles Darwin, hypothesized that the great rivers of the Amazon basin could be natural barriers affecting the diversity of life in the forest.
- While Wallace’s theory has been proven by studies of vertebrates, a new study now shows how it also applies to plant species.
- The study found that the great diversity of plants in the Amazon is not the result of one factor, but rather the result of a combination of many different factors.
- For some plants, wide rivers were an important barrier to the formation of new species; For others, dispersal of seeds by wind, water, and animals was the limiting factor.
Since the first European naturalists arrived in the Amazon rainforest at the end of the 18th century to catalog its abundant natural wonders, one of the most intriguing questions has been what created this rich biodiversity of flora and fauna in the region. The Amazon is home to the largest concentration of biodiversity on Earth, with 10% of the world’s described species within just 0.5% of the planet’s total surface area.
Among the scientists who were drawn to this area was British naturalist Alfred Russel Wallace, who developed his theory of natural selection around the same time that Charles Darwin was having his eureka moment in the Galapagos. Wallace explored the Amazon Basin from 1848 to 1852, coming up with the hypothesis that large rivers could act as natural barriers, influence geographic distribution, and generate an impressive gamut of organisms in the tropical rainforest – a biome with one hectare of land containing a variety It is a plant species like all of Europe.
When studying primates, Wallace noticed that some species found on one bank of the Negro River, a tributary of the Amazon, were not found on the opposite bank of the same river. He concluded that the Amazon’s very wide rivers, some of which can be up to 50 kilometers (30 miles) wide during the rainy season, may have limited the spread of animals between river banks and prevented gene flow, and the migration of genes between populations. The concept can be applied to different organisms to assess the level of genetic isolation among populations on opposite banks of rivers.
Practically speaking, Wallace’s hypothesis posits that, apart from separating groups of the same species, these geographic barriers over millions of years have led to the accumulation of genetic differences between groups located along the banks of different rivers, leading to the formation of entirely new species. . This is a phenomenon that occurs worldwide, particularly in mountainous regions such as the Andes, where the height of the mountain range has resulted in the great biodiversity found on both sides of the range today.
Wallace’s hypothesis has been demonstrated in studies focused on vertebrates, but has not yet been tested in terms of plant species. Now, more than a century and a half after Wallace invaded the Amazon, a group of Brazilian and American researchers set out to answer that question in a study recently published in the journal. Frontiers in plant sciences.
“We have reinterpreted Wallace’s hypothesis from a genetic perspective, where the original version addresses patterns of species distribution according to biogeographic barriers,” says lead author Alison Nazarino, professor at the Institute of Biological Sciences at the Federal University of Minas Gerais (UFMG). “Our study attempted to answer the question of what would be expected in these terms if a large river were to separate groups of the same species.”
The study aimed to expand the scope of the thesis by focusing on the genotype patterns in plants. The field trip that began the study lasted seven days and produced three years of work in the lab. On a boat on the Amazon River, researchers sailed the Negro River, which at some points can be more than 20 kilometers (12 miles) wide, to the mouth of the Branco River, one of its narrower tributaries, which is still 1 to 1 wide. km (0.6-2.5 mi) in the studied area. The goal was to test the hypothesis of rivers as levees by looking at rivers of different widths.
The researchers extracted DNA from collected leaf samples to estimate the level of gene flow between representative groups of four plant families: Bignoniaceae, Passifloraceae, Rubiaceae, and Violaceae.
“We measured genetic variation within a single plant species, but in populations on opposite banks of the river,” says Nazarino. We have used a genetic differentiation index that spans from zero to one. The measure closest to one indicates populations that, even if hypothetically crossed in the future, would not be able to leave their fertile descendants.”
The results showed that the vast diversity of Amazonian flora is not the result of any one factor but rather a combination of many historical and ecological drivers.
For some plant species, wide rivers are an important barrier to genetic relatedness, allowing the process of speciation, in which separate groups develop along different pathways. For others, environmental aspects such as seed dispersal by wind, water or animals along with pollination and adaptation to the soil are more relevant to their evolutionary history than geography. “The Branco River did not reflect any impediment to gene flow for any of the plant species we analyzed in this study,” says study co-author Lúcia Lohmann of the University of São Paulo’s Institute of Biosciences (IB-USP). On the other hand, the Negro River formed a great barrier to Longifolia Amphirrhox, a species in the family Violaceae”.
According to Lohmann, A. longifolia It is spread by fish that live in limited outlets that sometimes reach the other side of the river. This contributes to the genetic isolation of species groups in a process that is likely to give rise to new plant species in the future.
The same process of genetic isolation is evident in species Oxycarpa Buchenavia, of the Combretaceae plant family. The dispersal of monkeys that could not cross the Negro River, and groups of this species on the opposite banks of the river showed their own strong genetic structures, indicating limited gene flow – an important part of the speciation process.
Lohmann has coordinated studies on the origin and evolution of Amazonian organisms since 2003, along with Joel Kracraft of the American Museum of Natural History. She says this type of data is very important because it allows for the creation of a more efficient public policy that increases the preservation of key regions and lineages in the Amazon.
“Our knowledge of the Amazon is still quite fragmented. Only through management based on high-quality scientific knowledge and a solid familiarity with the ecological and evolutionary processes responsible for generating and maintaining the high level of biodiversity found in this region will we be able to devise effective strategies to conserve the great biodiversity that exists in the Amazon today,” says Lohmann.
“It is an investigative work that integrates different types of data – biological and geological – to discover the origin of the forest and the way it formed,” she adds. “Strong knowledge of the Amazon is key to its preservation, because humans only preserve what they know, love, and value.”
Nazarino, A. J., Knowles, L. L., Dick, C. W., & Loman, L. G. (2021). By animal, water, or wind: Can dispersal mode predict genetic relatedness in riverine plant species? Frontiers in plant sciencesAnd 12. doi: 10.3389/fpls.2021.626405
Banner photo: Researchers collect plant samples from a boat in the Amazon. Photo of Leonardo Ramos Chavez.
This story was reported by the Brazilian team Mongabay and was first published here on our website in Brazil on October 15, 2021.