The scientists focused on six groups of bird species that typically don’t fly very far. “If you want to know how the river affects birds, you also have to select the birds that the river affects,” Musher jokes. These species, including the blue-necked garlander (Galbula cyanicollis) and the Teardrop Coat Antbird (Phlegopsis nigromaculata), spend most of their time in the undergrowth of the southern Amazon lowlands. There they follow swarms of ants and eat insects that the ants start up.
The researchers sequenced the birds’ genes and compared them to each other. This enabled them to determine how these had changed over time. They then related the discovered changes in the genome to geological changes in the rivers around which the birds lived and still live. As expected, the research group found that rivers represent barriers for the birds: when rivers branched, populations were cut off from each other. Even relatively small rivers could separate populations from each other, promoting differences in their genomes. The scientists confirmed their findings with a model that uses the number of mutations in a species to predict how long ago it was separated from other species.
Always on the move
However, they also noticed that the river courses are dynamic rather than static barriers. Formerly divided rivers often rejoined, causing the separate populations to remix. Sometimes, after a long period of separation, the divergent populations were too different to interbreed again. From then on they remained separate species. As a rule, however, the birds used these reunions to exchange newly acquired genes. This “gene flow” resulted in new combinations of genes each time, and probably gave rise to a variety of new bird species over time, Musher explains.
In fact, the diversification of the different species varied according to how the rivers had changed and over what period of time. The researchers found that geology in the western Amazon basin caused greater gene flow between bird species than in the east. In the western Amazon, the landscape is rather flat and the rivers meander a lot more because their banks are eroding more, which changes the course of the river. In the east, on the other hand, the landscape is very hilly. Flowing waters dig into the rock here, which is why the river bed is usually much more stable and less winding.