Coleoptera is the largest order in the Animal Kingdom, comprising a quarter of all-known animal species. With more than 4200 described species, Dytiscidae (predaceous diving beetles) represents one of the largest and most diverse beetle families and is the most commonly encountered group of aquatic insects. In contrast to most of other insects, they have an adult stage that is truly aquatic resulting from a secondary return to the water environment during their evolution. They are occurring in various habitat types, where they form multi-species assemblages due to their high diversity and large variation in body size and habitat specificity.
We re-analyzed the molecular data from the recently published phylogeny of 164 Dytiscidae based on 9 DNA sequence fragments with the exclusion of highly variable regions and of the morphological characters. We dated the phylogeny with 12 fossils as a calibration and tested for shift in diversification rates and in body size. We also reconstructed the biogeographical history of the group using model-based likelihood.
This reanalyzed phylogeny revealed stronger support for important basal nodes. However, some relationships remain unsolved and call for a genomic approach to better resolve the evolution of Dytiscidae. The crown group of Dytiscidae was dated as ca. 211 Ma (Triassic) that is similar to the age recovered in the dating of the whole Coleoptera phylogeny (ca 219 Ma). The origin of the family was inferred as American with several independent dispersions to Palearctic and African regions. Despite the fact that the family is the most species rich among aquatic insects, we found no evidence for an acceleration of speciation during their evolution. However, the rate of morphological evolution, as represented by body size, revealed large heterogeneity among clades, showing that phenotypic differentiation has been decoupled from species diversification.