An Accidental Laboratory
When the Três Marias Dam was completed on the São Francisco River in 1961, it was an engineering feat designed for flood control and hydroelectric power. Construction began in 1957, employing 10,000 workers to build the 2,700-meter-long and 75-meter-high embankment. The result was one of Brazil's largest reservoirs, a body of water with a surface area of 1,040 square kilometers and a volume of 21 billion cubic meters. An unintended consequence of this massive project was the creation of a large-scale evolutionary experiment. The dam has no fish ladder, which effectively cut off fish migration along the upper São Francisco River. More importantly, the rising waters isolated the headwaters of numerous small tributaries, turning them into aquatic "islands." Fish populations that once interbred freely were now separated, leading to rapid genetic divergence.
For species like the small characin fish Astyanax lacustris, this fragmentation was a powerful evolutionary trigger. These fish, common throughout the basin, found themselves in small, isolated groups. With no gene flow between the newly separated streams, each population began its own independent evolutionary trajectory. Researchers have studied these isolated populations, using mitochondrial DNA to track the changes. Studies comparing different populations of Astyanax in the São Francisco basin have identified distinct genetic haplotypes, suggesting that isolation is driving divergence. While the exact rate of change within the reservoir's tributaries is a subject of ongoing research, the principle is clear: geographic isolation, even on the timescale of mere decades, can lead to measurable genetic differentiation.