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In a new study published in the journal Science Advances, an international research team led by the Leibniz Centre for Tropical Marine Research (ZMT) challenges traditional ideas about how new animal species form. The study focuses on hamlets – colourful reef fishes from the Caribbean. In addition to ZMT, scientists from the University of Oldenburg, the Senckenberg Research Institute and Natural History Museum Frankfurt and the Smithsonian Tropical Research Institute (STRI) in Panama were involved in the study. Researchers from Colombia, Mexico, the USA and the UK also contributed to the publication.
In his 1859 work On the Origin of Species, Charles Darwin recognised that the process of natural selection not only allows new species to form, but also results in a network of common ancestry. When a species splits into two new ones, these develop differences over time, but still continue to carry characteristics of their ancestors.
In this way, the lines of descent branch out further and further, similar to the branches of a tree. This results in groups of organisms that share a closer evolutionary relationship with one another than with other organisms. These relationships can be represented in the form of a family tree that shows how species are connected to each other through common ancestors.
Today, genetic differences are primarily used to investigate such relationships between organisms and to construct family trees. But when applying this approach to the 19 known species of hamlets (Hypoplectrus spp.), the researchers discovered something unexpected: despite their striking colour differences and their strong preference for mating with members of their own species, their family tree is genetically almost impossible to reconstruct.
“Most studies that explain how different species develop within a group start with a family tree based on genetic differences between species,” says Oscar Puebla, evolutionary scientist and fish ecologist at the Leibniz Centre for Tropical Marine Research (ZMT) in Bremen. “But in this case, there is only one genetic split among all 19 hamlet species that clearly separates the species.”
“We sequenced the entire genomes of 335 fish and were surprised to find that not a single gene allowed us to reconstruct a family tree for this group,” adds Martin Helmkampf, senior scientist at ZMT and one of the lead authors of the publication. “The genetic differences between the species are so small that even comparisons of the entire genome are insufficient. This challenges the common understanding of what species are and how they arise,” he says.
+++ New perspectives on speciation and biodiversity +++
Based on genetic data, the scientists identified only one gene that appears to be related to species differences. This gene, called casz1, is active in the cells of the skin, eyes and brain of hamlets and is thought to be involved in the formation and perception of colour patterns that play a role in mate selection.
However, even this gene did not allow researchers to reconstruct a family tree for the group. “This is probably because the species differences are influenced by many genes together and in different combinations, making it impossible in some cases to reconstruct a family tree that distinguishes all species from each other,” explains former ZMT researcher Floriane Coulmance, who is also a lead author of the publication.
Co-author Iliana Bista from the Senckenberg Research Institute and Natural History Museum Frankfurt adds: “The unique case of the hamlet fish radiation highlights the importance of acquiring extensive and complete genomic datasets in order to fully characterise the evolution of taxa with unusual evolutionary paths, without which we would not be able to see the full picture.”
Oscar Puebla summarises: “The results of the study challenge the view that speciation always leaves clear genetic traces and occurs like a family tree through simple splitting processes. Instead, new species can evolve very quickly and through changes in just a handful of important genes. This is remarkable evidence of how evolution works and how biodiversity arises on our planet.”
Dr. Martin Helmkampf | Senior Scientist | Working Group Fish Ecology and Evolution | Leibniz Centre for Tropical Marine Research (ZMT)
Email: martin.helmkampf@leibniz-zmt.de
Prof. Dr. Oscar Pubela | Head of Working Group Fish Ecology and Evolution | Leibniz Centre for Tropical Marine Research (ZMT)
Email: oscar.puebla@leibniz-zmt.de
Helmkampf M, Coulmance F, Heckwolf MJ, Acero A, Balard A, Bista I, Dominguez O, Frandsen PB, Torres-Oliva M, Santaquieteria A, Tavera J, Victor BC, Robertson DR, Betancur-R R, McMillan WO, Puebla O. 2025. Radiation with reproductive isolation in the near absence of phylogenetic signal. Science Advances, 11, eadt0973, DOI: https://doi.org/10.1126/sciadv.adt0973.
Barred Hamlet (Hypoplectrus puella), Bocas del Toro Archipelago, Panama
Copyright: Floriane Coulmance, Leibniz Centre for Tropical Marine Research (ZMT)
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Barred Hamlet (Hypoplectrus puella), Bocas del Toro Archipelago, Panama
Copyright: Floriane Coulmance, Leibniz Centre for Tropical Marine Research (ZMT)
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