This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: My students and I encountered a mystery. We were partnering with the Missouri Department of Conservation to monitor for fish presence and abundance in the Missouri Ozarks.
We visited half a dozen rivers and streams across the region and collected a few cups of water from each. We were testing for environmental DNA to determine which species had left behind their genetic material and were therefore present in the stream. There was no need to capture or even see the species themselves.
We were delighted to detect a variety of species in their appropriate habitats that are considered at risk in Missouri, including the lake sturgeon, crystal darter and Alabama shad. In the surface waters emerging from one large underground spring, we detected DNA from a cave-dwelling fish species that would never have been detected by researchers who caught or looked only for fish in the surface waters. But there was a problem: We were certain we would find one particular species in every stream: the bleeding shiner (Luxilus zonatus), one of the most common fish in Ozark streams.
To have confidence in our results, we needed to understand why the bleeding shiner DNA did not show up in every sample, as we had expected. We checked our sampling technique, our water chemistry and other aspects of our testing. In the data itself, we found a clue: The streams in which we detected bleeding shiner DNA all drained to the southern Ozarks, and the streams in which bleeding shiner DNA was not detected were all in the northern Ozarks.
We could draw a line through the north-south drainage divide between those areas. Some follow-up sleuthing revealed something we hadn't known at the start: Northern Ozark bleeding shiners have a different DNA history from their southern relatives, even though the fish look identical and are, in fact, the same scientific species. To human observers, the Ozarks can seem a relatively continuous landscape.
For fish, however, that landscape is divided into a series of separate drainage networks. Streams may arise only a few miles apart yet flow in opposite directions, ultimately connecting to entirely different river systems with distinct geologic histories. Hundreds of thousands of years ago, the ancestors of northern populations interbred with another species of shiner, leaving behind a genetic signature of ancient hybridization that survives today, but only in certain drainages.
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