Researchers lay a seine net, anchored by bottom weights, across a creak. Upstream, a team member walks toward it, kicking up the bottom ground. Small fish a few inches long, ground feeders, panic at their approach, and some get caught in a current. They sweep into the the net. This fieldwork, done in all waterways over the east half of North America, has provided Dan with his data. He uses collections, assembled for the sake of cataloging, to look at bigger questions of what makes a species a species, and how they change.
Dan studies freshwater fish called darters: the johhny darter, Etheostoma nigrum, and the tessellated darter, Etheostoma olmstedi. Among North America’s two hundred and fifty species of darters, these two are the most widespread. Their ranges stretch from the southern Appalachians up to and, in the case of the johnny darter, past the Canadian border. Where those ranges overlap, which occurs in three places, the two species interbreed. The resulting hybrids raise a question.
“They’re sister species, which makes this kind of a cool system in which to ask, if they can hybridize, how are they maintaining their identity as separate species?” This is Dan’s research focus.
Why are they not one species? Why have they not, reproducing over millennia, slowly melded into a single group? Hybridization breaks down the barriers that scientists are constantly trying to define. And in fact, over the last century, scientists have lumped and split, combined and re-divided, both in messy taxonomic history. Now, their definitions are based on very small physical features, like the number of fin rays in the first pectoral fin. “Twelve or thirteen, that is enough to separate species,” Dan said. It takes effort to tell the difference. Dan’s work asks what that difference looks like in the genetic code.
The project is part of a bigger renaissance of the scientific art of identification. In recent decades, cataloging, naming species, fell out of popularity. But now old species definitions are called into question by new techniques: genomic sequencing. Dan explained. “Often there is conflict between what the morphology says and what the genetic data say.” Two lines of evidence, the physical and the genetic, sometimes contradict each other. The challenge is to reconcile them, and these darters are well suited to the task.
They are widespread and small. Thus their range is wide, which creates many opportunities for sampling, and, as a result of their size, they can reproduce quickly, swiftly exchanging genes. The darters are also easy to catch. Dan has gone out on four field collecting trips so far. “We drive around in a big pick up truck and hop out at bridges.” Set up the seine net, gather their samples.
But if Dan was collecting all his samples himself, he’d never have a study. His field work fills in research gaps from a much more traditional source: natural history museums, including the Yale Peabody. Behind the displays and public exhibits, these institutions exist, in large part, to catalog biodiversity. And museums across the darters’ range have preserved thousands of the finger-length fish. Dan pointed to work by North Carolina Museum of Natural Sciences, which put together a road trip through that state and Virginia, cataloging as much biology as possible. He wrote to them, asking about darters, and they sent back a list of three hundred samples. The samples themselves testify to the museum’s work.
“You can actually follow their trip by looking at all these locations, where they stopped and the dates. They sampled in pretty much every major river that they could access,“ Dan said.
These trips provide the data for the science to follow. Dan, and his peers, are indebted.
Dan’s study, which will be based on genetic material from roughly seven hundred of the small fish, will help him start answering broad evolutionary questions. Perhaps the two species were separated and only recently came back together. Perhaps they have always been exchanging genes but still, somehow, maintained themselves as separate species. He elaborated.
“How have they been able to become two separate species even in the face of this melding, this gene flow they’re experiencing? It doesn’t matter whether you work on fish or plants or mammals, how species form, that question is something that everyone is interested in.”
Knowing what things are is the first step to mapping them across the landscape, identifying their roles in ecosystems, even following their populations through time. “It’s absolutely crucial that we accurately describe and delimit species.” If two species get lumped together, Dan pointed out, then an animal in decline wouldn’t get the protection it needed. Only named species can be listed as threatened or endangered. Name it, save it.
Understanding how species interact with each other in their meeting places tells you about their history of separation—and establishes the groundwork for all the science and conservation to follow. We need to name before we study. Dan starts with the fish.
Dan Macguigan’s study was supported by a YIBS pilot grant.
Edited by Juliana Hanle