Beetle evolution illuminates the geological history of the world’s most diverse tropical archipelagoSubmitted by editor on 3 December 2023. Get the paper!
Figure 1. One hundred species of the genus Trigonopterus. Credit: Alexander Riedel.
The islands of Indonesia and the West Pacific host an extraordinary diversity of life, including more than 1000 species of flightless Trigonopterus beetles. Using statistical analyses of DNA sequences from 1006 Trigonopterus species, we showed that beetle evolution has tracked the formation of terrestrial habitats in Indonesia and the West Pacific over the past 40 million years, revealing new insights into the complex geological history of this tropical region and the origins and evolution of its exceptional and threatened biodiversity. The study was published open access in the journal Ecography.
The world´s largest tropical archipelago, comprising Malaysia, Indonesia, the Philippines, Papua New Guinea, and other island nations of the West Pacific, is well known for its extraordinary biodiversity and complex geological history. Alfred Russel Wallace famously studied the fauna of this region, resulting in his independent discovery of the theory of evolution and his foundational contributions to biogeography, a discipline that explores the geographic distributions of organisms and the factors shaping such patterns. Wallace largely focused on conspicuous groups of animals, such as birds and butterflies, as was customary at the time. However, he also collected beetles, including a few tiny weevils from the exceptionally diverse genus Trigonopterus.
Figure 2. Schematic evolutionary history of 1006 Trigonopterus species. Credit: Harald Letsch.
Trigonopterus beetles are wingless and, thus, flightless, confining most species to relatively small geographic areas. Alexander Riedel (Natural History Museum Karlsruhe) has undertaken extensive field studies of Trigonopterus across their geographic range in Southeast Asia, Australia, and the West Pacific, documenting hundreds of undescribed species. According to Riedel: “Having named more than 370 new species and with many additional new species at hand, we decided it was time to look at the big picture and examine the evolutionary history of all Trigonopterus species.”
Riedel´s co-worker Harald Letsch (University of Vienna) generated a time-calibrated phylogeny (genealogical tree of Trigonopterus species), based on DNA data from 1006 species. This is not only one of the largest phylogenies ever produced for a single genus of animals but also reveals new insights into the complex geological history of Indonesia and the West Pacific and the origins and evolution of its exceptional and threatened biodiversity.
Notably, Letsch et al.’s work showed that Trigonopterus weevils experienced relatively few dispersal events in their 40 million-year history, indicating that their phylogenetic tree should contain strong signatures of regional geological history, including the formation and erosion of islands and their topographic features. Geographic barriers to dispersal and geographic isolation tend to cause populations of poorly dispersing species, like flightless Trigonopterus beetles, to diverge and form different species. In an archipelagic setting, increases in rates of diversification suggest the presence and possible increases in the extent or topographic complexity of habitable land, regardless of whether that land still exists. Thus, biological data like those reported on weevils can provide information on the geological history of a landscape, even where physical evidence has been lost to erosion.
Figure 3. Schematic illustration of the dispersal of Trigonopterus weevils out of northern Australia. Credit: Harald Letsch.
Maps of Indonesia and the West Pacific islands over the last 40 million years reconstructed by Letsch et al. based on the evolutionary history of Trigonopterus beetles provide intriguing new insights into the enigmatic geological history of the region. For example, they suggest that the Papuan Peninsula of New Guinea was emergent by ca. 40 million years ago, earlier than some existing hypotheses. Additionally, the diversification of Trigonopterus on the Samoan Islands is indicated to have started ca 23 million years ago, suggesting the existence of eroded and now sunken islands far to the west of the current island chain. Thus, Letsch et al. notably demonstrate that their approach to reconstructing regional geological history can be of great value to geology, emancipating the discipline of biogeography from its role as a net receiver of information to a contributor of useful data to related disciplines.
Figure 4. Virgin rainforest, the habitat of Trigonopterus weevils, here on the island of Biak. Credit: Alexander Riedel.
Letsch, H., Balke, M., Kusy, D., McKenna, D.D., Narakusumo, R.D., Sagata, K., Toussaint, E.F.A., White, L.T., Riedel, A. 2023. Beetle evolution illuminates the geological history of the world’s most diverse tropical archipelago. Ecography 2023: e06898.
Alexander Riedel (State Museum of Natural History Karlsruhe, Germany)
Email: riedel [at] smnk [dot] de
Harald Letsch (University of Vienna, Austria)
Email: harald [dot] letsch [at] univie [dot] ac [dot] at
Duane D. McKenna (University of Memphis, USA)
Email: dmckenna [at] memphis [dot] edu