14 DAY TRIAL // Understanding how animals become different species, a process known as speciation, is a major concern in understanding evolution. The most common type of speciation is geographically dependent: animals diverge into separate species because they become physically isolated from each other and eventually genes within each group mutate so that the groups can no longer be considered to be of the same species.
Another type of speciation, which many scientists have found harder to imagine, involves animals that live in the same geographic location but, for some reason, begin to mate selectively and form distinct groups and, ultimately, separate species. This so-called sympatric speciation is more controversial because there have been few accepted examples of it to date.
But now in the banks of the Ivindo River in Gabon, Cornell neurobiologists may have just found an ongoing example of sympatric speciation. They have found a certain species of electric fish that produce two different types of electric signals. Although the two types of electric signals come from fish that have the same DNA the fish don't seem to interbreed with each other. Thus, they appear to be on the verge of forming two separate species.
"We think we are seeing evolution in action," said Matt Arnegard, a neurobiology postdoctoral researcher in the laboratory of Carl Hopkins, Cornell professor of neurobiology and behavior, who has been recording electric fish in Gabon since the 1970s.
The electric fish -- known as mormyrids -- emit weak electric fields from a batterylike organ in their Tails to sense their surroundings and communicate with other fish. Each species of mormyrid gives off a single characteristic electric impulse resulting in the flash of signals, indicating, for example, aggression, courtship and fear. While the fish may be able to understand other species' impulses, said Arnegard, "they seem to only choose to mate with other fish having the same signature waveform as their own."
But when Arnegard decided to take a genetic look at these particular fish, he couldn't find any differences in their DNA sequences. "These fish have different signals and different appearances, so we were surprised to find no detectable variation in the genetic markers we studied," he said. Because all of the 20 or so species of mormyrid have distinct electric signals, Arnegard believes the different impulses of the fish he studies might be their first step in diverging into different species. "This might be a snapshot of evolution," he said.