The structure of the Cardiophorinae-Dendrometrinae clade (Coleoptera: Elateridae) is the weakest point of the current click-beetle phylogenetics and classification

In the natural world, the history of life is written in the genes of every living creature, but reading that history is often like trying to reconstruct a shattered vase when some of the pieces are missing and others look suspiciously similar. For scientists who study how species are related, a group of insects known as click beetles has long presented a particularly stubborn puzzle. These beetles, famous for their ability to snap their bodies and flip into the air to escape predators, belong to a family called Elateridae. Within this family, researchers have spent decades trying to sort out the relationships between several major groups, or subfamilies, that seem to have evolved very rapidly and very close together in time. When evolution happens this quickly, the genetic signals that usually tell scientists who is related to whom become blurred and contradictory. Different studies using different methods have produced conflicting family trees, leaving the classification of these beetles in a state of confusion. Without a clear map of their evolutionary history, it is difficult to understand how these insects diversified or to name them in a way that reflects their true biological connections.

A team of researchers from Palacký University in the Czech Republic has now tackled this specific problem by looking at the genetic code of these beetles with unprecedented depth. Instead of relying on small snippets of DNA or physical features alone, they analyzed thousands of genetic markers from the entire genomes of thirteen species representing the most contentious groups. Their goal was to cut through the noise of conflicting data to find the parts of the evolutionary story that are actually clear. What they found was a mix of certainty and uncertainty. On one hand, they confirmed that five distinct groups of beetles are indeed real, separate lineages that have existed for millions of years. On the other hand, they discovered that the exact order in which these five groups split from one another is so deeply scrambled by rapid ancient evolution that no amount of genetic data can currently resolve it. The researchers concluded that the current way these beetles are classified is fundamentally flawed because it tries to force a clear, branching tree onto a part of history that simply does not have one.

The study focused on a specific cluster of click beetle subfamilies: Dendrometrinae, Hypnoidinae, Cardiophorinae, and Negastriinae. For years, scientists have argued over whether these groups are all part of one large family or if they should be split up. Previous studies using older genetic techniques or physical characteristics had produced a dozen different possible family trees, none of which agreed with each other. Some suggested that one group was the sister to another, while others placed them in completely different spots. This lack of agreement meant that the scientific names used for these beetles were unstable, changing every time a new study was published. The researchers decided to test these competing ideas using a massive dataset of genetic information, analyzing over four thousand distinct genes from each beetle. They used two different statistical approaches to build their trees: one that looks at the combined genetic signal as a whole, and another that accounts for the fact that individual genes can have their own unique histories due to random biological events.

The results revealed a clear pattern of what is known and what remains unknowable. The researchers confirmed that five specific groups are solid, distinct lineages. These include the Cardiophorinae and Negastriinae, which have long been recognized as separate families, and three other groups that had been lumped together under the name Dendrometrinae. The study showed that the group formerly known as Dendrometrinae is actually a mix of unrelated lineages. Specifically, the researchers found that the tribes known as Dimini, Plastocerini, and Oxynopterini are not closely related to the core Dendrometrinae group as previously thought. Instead, they form their own distinct branch. Similarly, the tribe Prosternini stands apart from the others. The only relationship that appeared consistently across all their analyses was a strong partnership between the Cardiophorinae and Negastriinae, which always appeared as a single, united pair.

However, when the researchers tried to figure out how these five solid groups fit together relative to one another, the data became a blur. The genetic signals for the branching points between these groups were so weak and contradictory that the computer models could not decide on a single answer. In some analyses, one group appeared to be the closest relative to the others; in others, a different group took that spot. The researchers determined that this confusion is not a failure of their technology or their methods, but a genuine feature of the beetles' history. It appears that these groups evolved in a very rapid burst, leaving very little time for unique genetic changes to accumulate between each split. This phenomenon, known as incomplete lineage sorting, means that the genetic history of these beetles is not a clean, branching tree but rather a tangled web where the lines of ancestry cross over one another. Because the signal is so noisy, trying to force a single, definitive family tree onto this part of the evolutionary history would be misleading.

Given this biological reality, the authors propose a new classification system that prioritizes stability and accuracy over trying to guess the exact order of ancient splits. They recommend abandoning the old, broad definition of Dendrometrinae, which they describe as an unsupported and likely unnatural grouping. Instead, they suggest elevating the distinct lineages to the rank of subfamily. This means recognizing Oxynopterinae as a separate subfamily that includes the tribes Dimini, Plastocerini, and Oxynopterini. They also propose raising Prosterninae to a full subfamily rank. The group Hypnoidinae retains its status as a subfamily, as its position has been debated as either a sister to the Dendrometrinae or to the Cardiophorinae plus Negastriinae pair. The core Dendrometrinae is redefined to include only the tribes Selatosomini, Dendrometrini, and Pleonomini. This new system acknowledges that while we cannot yet say exactly how these five groups are related to each other, we can be certain that they are five separate, major branches of the click beetle family tree.

The researchers emphasize that this change is not about splitting the beetles into too many small groups, but about creating a classification that reflects what the data actually supports. The entire cluster contains about four thousand described species, and the new subfamilies range from the small Hypnoidinae with about one hundred species to the large Dendrometrinae with over sixteen hundred. By giving each of these robust lineages its own subfamily rank, the classification becomes stable and resistant to future changes, even as new data comes in. The study also notes that for one small group, the Crepidomenini, the researchers could not make a final decision because they lacked the necessary genetic data. These are currently left in a temporary holding category until more information is available. Ultimately, the paper concludes that the structure of this part of the click beetle family tree is the weakest point in current understanding, not because of a lack of effort, but because the evolutionary history itself was too rapid and chaotic to leave a clear genetic trail. The proposed solution is to accept this uncertainty and build a classification that only names the parts of the tree that are undeniably real.