When a new species is discovered, it's tempting to imagine an adventure novel, said Chan Kin Onn of Michigan State University.
"Most people have this image of an intrepid explorer braving an isolated mountain or some other remote place, and stumbling across a creature that no one has ever seen before," Chan said.
Sure, that still happens occasionally. "But most of the time it's far less glamorous," he added.
Instead, the vast majority of new vertebrate species are "discovered" by revisiting known populations with new data or tools, and showing they were more distinct than previously thought.
Chan is a herpetologist, a scientist who specializes in studying amphibians and reptiles like frogs, turtles, lizards, and snakes. There are more than 9,000 species of amphibians on the planet, and each year roughly 100 to 200 are added to the list, he said.
Take a group of little brown frogs from Southeast Asia called the Bornean fanged frogs, so called because of tooth-like projections on their jaws. One of them, Limnonectes kuhlii, has been known to science since 1838. But in the last two decades, genetic analyses have found that what looks like one species might actually be as many as 18.
"Animals that look similar but are genetically distinct are called cryptic species," said Chan, who is also Curator of Vertebrate Collections and a core faculty member in MSU's Ecology, Evolution and Behavior program.
Due to advances in genetic sequencing, "a ton of cryptic species are being discovered left and right."
To see if, in fact, these frogs had been woefully undercounted, Chan and colleagues extracted DNA from specimens collected across the mountain rainforests of Malaysian Borneo and analyzed more than 13,000 genes across their genomes.
According to their work, published Jan. 14 in the journal Systematic Biology, the frogs do indeed fall into multiple genetic clusters. But only six or seven clusters could be classified as distinct species.
"It's not just one species. But it's not 18 species, either," Chan said.
The question is more than an academic hairsplitting exercise.
That's because the world's frogs are in trouble. A 2023 study of some 8,000 amphibian species worldwide revealed that two out of five amphibian species are threatened with extinction, making them the most endangered group of vertebrates on the planet.
On the one hand, if we don't know a species exists, we can't protect it, said Chan, who was a co-author on the 2023 study.
"There are so many species in the world that we still haven't discovered, and that could go extinct before we can give them a name," Chan said.
"But there's a flip side to that coin too," he added.
Overzealously splitting what was once considered one species into multiples can create problems for conservation biologists, making the geographic range of newly described cryptic species seem more restricted — and their situation more dire — than it really is.
"We cannot possibly conserve everything, so we have to triage and decide how to allocate limited resources towards what we think are the highest priorities," Chan said. "We could be putting names on things that shouldn't be prioritized."
The researchers also found a lot of interbreeding between these different frogs.
"We found a ton of gene flow going on," Chan said.
All the DNA moving back and forth can make for blurry dividing lines. As a result, some of the growing number of cryptic species may be more methodological artifact than biological reality, he added.
The fanged frogs in Borneo show that species don't evolve instantaneously. "It's not like all of a sudden, boom. It's more of a continuum," Chan said.
Fanged frogs are by no means the only group of animals whose numbers scientists may have miscalculated.
Over the past two decades, genetic studies of animals ranging from insects and fish to birds and mammals suggest there may be a staggering number of species hiding in plain sight.
Where once the total number of species on Earth was thought to be 8.7 million, more recent models accounting for cryptic species suggest the true number may be anywhere from 7 to 250 times that.
So where does the true number lie? "This study shows that there's a speciation 'gray zone' that can make it hard to draw the line," Chan said.
This research was supported by grants from the U.S. National Science Foundation (GRFP 1540502, 1451148, 0907996 and 1654388).
CITATION: "A Genomic Perspective on Cryptic Species Reveals Complex Evolutionary Dynamics in the Gray Zone of the Speciation Continuum," Kin Onn Chan, Dario N Neokleous, Shahrul Anuar, Rafe M Brown, Carl R Hutter, Indraneil Das, Stefan T Hertwig. Systematic Biology, Jan. 14, 2026. DOI: 10.1093/sysbio/syag001
