A new study led by researchers at the University of Arizona suggests that for every recognized vertebrate species, there are, on average, two additional species that have gone unnoticed. These overlooked organisms, known as “cryptic” species, appear almost identical to known species but are genetically distinct. The findings indicate that global vertebrate biodiversity may be far greater than current estimates, raising important questions about how many species remain undocumented and unprotected.
“Each species that you and I can see and recognize as distinct may actually be hiding two different species, on average,” said John Wiens, senior author of the paper and a professor in the University of Arizona Department of Ecology and Evolutionary Biology in the College of Science. “This means that across vertebrates, there may be twice as many species as we previously thought, and many of these hidden species could already be at risk of extinction.”
DNA Reveals Cryptic Species Hidden in Plain Sight
Traditionally, scientists have identified and classified animals based on visible physical traits, also known as morphological features. Differences in color patterns, scale arrangements, or body shape often separate one species from another. For instance, snake species may be distinguished by variations in their markings or build.
Cryptic species complicate that process. Although they look nearly identical, genetic analysis shows they belong to separate evolutionary lineages. In other words, their DNA reveals differences that cannot be seen with the naked eye.
“Many of these cryptic species have likely been evolving separately for a million years or more,” said Wiens. “So, their DNA tells us that they’ve been distinct for a long time, even if they look identical.”
Consistent Pattern Across Fish, Birds, Mammals, and More
Advances in molecular sequencing have made it faster and more affordable to compare DNA among populations. As genetic data have accumulated, researchers have repeatedly uncovered previously unrecognized species.
What surprised the team most was how widespread this pattern appears to be. According to Wiens, the trend holds across major vertebrate groups. “On average, morphologically based species of fishes, birds, mammals, reptiles, amphibians and other vertebrate groups all seemed to be hiding around two cryptic species.”
A striking example comes from Arizona. For years, the Arizona mountain kingsnake was considered a single species throughout the state because individuals looked alike. However, molecular research in 2011 showed that snakes from Northern Arizona were genetically distinct from those in the south. As a result, the southern population was elevated to full species status as Lampropeltis knoblochi, while the northern snakes retained the name Lampropeltis pyromelana.
“If you compare those two mountain kingsnakes, they all look pretty much the same with their red, black and yellow-white stripes,” said Yinpeng Zhang, a graduate student in the Department of Ecology and Evolutionary Biology and the paper’s first author. “But the molecular data show that there are distinct but cryptic northern and southern species.”
Analyzing Hundreds of Studies Worldwide
Zhang began exploring the idea several years ago after noticing that many taxonomy studies were uncovering genetically distinct species that were visually indistinguishable. He realized that no one had systematically examined how common this phenomenon might be across vertebrates as a whole.
To answer that question, the research team compiled and analyzed findings from more than three hundred published studies from around the world.
“There aren’t many research groups focused purely on cryptic species,” Zhang said. “Most people discover them as a byproduct of other biodiversity or taxonomy studies rather than as the main goal.”
The researchers also evaluated different approaches used to estimate the number of cryptic species, offering guidance for future research in this area.
Conservation Risks for Newly Recognized Species
The implications extend beyond classification. If what was once considered a single widespread species is split into several distinct cryptic species, each newly identified species occupies a smaller geographic range. That can significantly increase its vulnerability.
“People have generally found that the smaller a species’ range size is, the more likely that species is to go extinct,” said Wiens.
Wiens argues that formally describing and naming these species is a critical first step toward protecting them.
“Even though hundreds of molecular studies have uncovered hundreds of cryptic species, very few have been formally described or named,” said Wiens. “That leaves these species without official recognition or legal protection.”
There are also practical consequences. According to Zhang, conservation programs aimed at increasing population numbers could accidentally breed individuals from different species if cryptic species are not properly identified.
“Hidden diversity is an important consideration to make in our conservation efforts,” Zhang said.
For the researchers, the takeaway is straightforward.
“If we don’t know a species exists, then we can’t protect it,” Wiens said.
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