The large lakes of East Africa have been called “Darwin’s dream ponds”—bodies of water filled with astonishing numbers of fish species that fascinate evolutionary scientists. But how do fish that live together in the same place spin off into new species? Researchers think they’ve found the answer in a nearby lake they characterize as more of a puddle.

In a paper published today in Science, biologists describe a small crater lake in Tanzania where they say the fish are in the process of sympatric speciation, or the process by which new species evolve from a single ancestor while living in the same geographic area.

The concept is so controversial that some scientists don’t believe it really happens. To become a new species, organisms must become different enough so that they cannot breed with organisms of other species. Usually this happens when a segment of one population is somehow removed from others of its kind. In a new location, a reduced gene pool or different environmental pressures can cause an entirely different species to develop.

But when animals live in close proximity, they share the same habitat and environmental pressures. Since there are no obvious physical barriers between the animals, they could hypothetically exchange genetic material with one another and have no need to differentiate. That has biologists struggling to figure out why new species sometimes do emerge in small spaces.

In the new paper, the research team studied the genomes of 146 cichlid fish living in the crater lake. The small lake contains two kinds of fish: one with yellow-green males that prefer the shallows and another with blue-black males that prefer the depths. However, the fish still often swim in the same places, with no barriers to their interactions.

The team discovered tiny genetic differences between the two forms of fish. Their study revealed that less than one percent of the fishes’ genes are different—primarily those that pertain to light sensing and shape.

The research team thinks that even though the animals swim together, look largely similar and are nearly identical genetically, they seem to prefer different mates, suggesting they are separate species. But this example of sympatric speciation in action doesn’t mean the concept has been definitively proven.

Now it’s up to scientists to look further into the mechanisms of how those sex preferences develop in a simple system like the crater lake—and how the processes might happen elsewhere. “These fish have much to tell us,” co-author Martin Genner of the University of Bristol says in a press release.