Since the first European naturalists arrived in the Amazon Rainforest at the end of the 18th century to catalog its copious natural wonders, one of the most intriguing questions has been what created such rich biodiversity of flora and fauna in the region. The Amazon is home to the greatest concentration of biodiversity on Earth, with 10% of the world’s described species inside just 0.5% of the planet’s total surface area.

Among the scientists drawn to this region was the British naturalist Alfred Russel Wallace, who developed his theory of natural selection around the same time that Charles Darwin was having his own eureka moment in the Galápagos. Wallace explored the Amazon Basin from 1848 to 1852, coming up with the hypothesis that large rivers could serve as natural barriers and influence the geographic distribution and generation of the splendid gamut of living beings in a tropical rainforest — a biome in which a single hectare of land holds as much variety of plant species as 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 immensely broad rivers in the Amazon, some of which can reach widths of 50 kilometers (30 miles) during the rainy season, may have limited the dispersal of animals between riverbanks and blocked genetic flow, the migration of genes between populations. The concept can be applied to different organisms to evaluate the level of genetic isolation between populations located on the opposite banks of rivers.

In practical terms, Wallace’s hypothesis postulated that, aside from separating populations of the same species, over millions of years these geographic barriers would have led to an accumulation of genetic differences among the groups located along different riverbanks, leading to the formation of entirely new species. This is a phenomenon that occurs all over the world, especially in mountainous regions like the Andes, where the uplift of the mountain range generated the great biodiversity found on either side of the range today.

Wallace’s hypothesis was proven in studies focused on vertebrates, but had still not been tested in terms of plant species. Now, more than a century and a half since Wallace’s foray in the Amazon, a group of Brazilian and U.S. researchers have set out to answer that question in a study recently published in the journal Frontiers in Plant Science.

“We reinterpreted Wallace’s hypothesis in genetic terms, as the original version addresses species distribution patterns according to biogeographical barriers,” says lead author Alison Nazareno, a professor at the Institute of Biological Sciences at Minas Gerais Federal University (UFMG). “Our study tried to answer the question of what could be expected in these terms if a large river were to separate populations of the same species.”

The study aimed to expand the thesis by focusing on genetic structuring patterns in plants. The field expedition that started off the study lasted for seven days and generated three years’ worth of work in the laboratory. Aboard an Amazon riverboat, the researchers navigated the Negro River, which at some points can reach a width of more than 20 km (12 mi), to the mouth of the Branco River, one of its narrower tributaries, which still ranges from 1-4 km (0.6-2.5 mi) wide in the area studied. The goal was to test the hypothesis of rivers as barriers by looking at rivers of different widths.

The researchers extracted DNA from collected leaf samples to estimate the level of genetic flow between representative populations of four botanical families: Bignoniaceae, Passifloraceae, Rubiaceae and Violaceae.

“We measured the genetic variation within single plant species, but in populations located on opposite banks of the river,” Nazareno says. “We used a genetic differentiation index that runs from zero to one. The scale closest to one indicates populations that, even if they were to hypothetically cross in the future, would not manage to leave fertile descendants.”

The results showed that the wide variety of Amazonian flora is not the result of any single factor but rather a combination of many historical and ecological drivers.

For some plant species, wide rivers are an important barrier to genetic connectedness, thereby allowing the process of speciation, where separate populations evolve along different trajectories. For others, ecological aspects such as seed dispersal via wind, water or animals together with pollination and adaptation to the soil are more relevant for their evolutionary history than geography. “The Branco River reflected no barrier to the genetic flow of any of the plant species we analyzed in this study,” says study co-author Lúcia Lohmann from the Biosciences Institute at the University of São Paulo (IB-USP). “On the other hand, the Negro River posed a significant barrier for Amphirrhox longifolia, a species in the Violaceae family.”

According to Lohmann, A. longifolia is disseminated by fish living in restricted niches and that occasionally reach the opposite side of the river. This contributes to the genetic isolation of populations of the species in a process that will likely lead to the emergence of new plant species in the future.

The same genetic isolation process is evident in the species Buchenavia oxycarpa, from the Combretaceae plant family. Dispersed by primates that cannot cross the Negro River, populations of this species found on opposite banks of the river showed a strong genetic structures of their own, indicating limited gene flow — an important part of the speciation process.

Lohmann has coordinated studies on the origin and evolution of Amazonian biota since 2003, together with Joel Cracraft from the American Museum of Natural History. She says this type of data is quite important because it allows for the creation of more efficient public policy that maximizes conservation of key regions and lineages in the Amazon.

“Our knowledge of the Amazon is still quite fragmented. It is only through management based on high-quality scientific knowledge and strong familiarity with the ecological and evolutionary processes responsible for generating and maintaining the high level of biodiversity found in this region that we will be able to establish effective strategies for conservation of the great biodiversity found in the Amazon today,” Lohmann says.

“It’s detective work that integrates different types of data — both biological and geological — to discover the forest’s origin and the way it was formed,” she adds. “Strong knowledge of the Amazon is key to its preservation, since humans only preserve what they know, like and value.”

Citation:

Nazareno, A. G., Knowles, L. L., Dick, C. W., & Lohmann, L. G. (2021). By animal, water, or wind: Can dispersal mode predict genetic connectivity in riverine plant species? Frontiers in Plant Science, 12. doi:10.3389/fpls.2021.626405

Banner image: Researchers collect plant samples from a boat in the Amazon. Image by Leonardo Ramos Chaves.

This story was reported by Mongabay’s Brazil team and first published here on our Brazil site on Oct. 15, 2021.