Scientists are watching a new species evolve before their eyes — even as the original species evolves in another direction.

A new study published in PLOS Genetics by researchers from Eawag and the University of Bern examines the rapid evolution of new species of threespine stickleback in a Swiss lake, reported Phys.org.

The fish are considered a nuisance by fisherman in Lake Constance, and they appear to be successful in various parts of the lake, which is broken into three main bodies of water, regardless of environmental factors.

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That’s because they can evolve so rapidly to adapt to new environments they encounter in the lake, which is on the Rhine River in the Swiss Alps.

An elaborate genetic study found two forms of the fish — one that thrives in the lake itself and another adapted to inflowing streams.

Lake stickleback migrate into those streams during spawning season, yet the two distinct forms have evolved away from one another in a short period of time.

Independent species typically develop by adapting to separate habitats and reproducing in isolation — but the new variation of the stickleback evolved alongside its original variant.

“It was completely unexpected for the species to diverge over such a short period, given that the sticklebacks breed at the same time and at the same sites,” said the study’s lead author, David Marques.

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The researchers said the rapid evolution serves as a model for how other species diverged into two or more distinct species.