The effect of habitat type on speciation rates and range movements in aquatic beetles: inferences from species-level phylogenies

Most aquatic beetles in the family Dytiscidae are tightly associated either with running (lotic) or stagnant (lentic) water bodies. The range size of lotic species is known to be, on average, much smaller than that of lentic species, presumably as a result of differences in dispersal strategies in e ach habitat type. We explored possible effects of these differences on clad e evolution and speciation rates by comparing species-level phylogenies bas ed on cytochrome oxidase I (COI) and 16S rRNA mitochondrial genes for two g enera, the lentic Ilybius and the lotic Deronectes. The expectation that sp ecies turnover is higher in lotic lineages due to their lower dispersal pro pensity compared to lentic species was not strongly supported. Deronectes d isplays a higher frequency of recent splits than Ilybius, consistent with t he hypothesis, but the difference was not significant compared to expected patterns under a constant speciation rate null model. Similarly, when the d egree of sympatry was plotted against relative node age, more allopatric sp lits were evident in the lentic Deronectes, suggesting a slower rate of ran ge movement since speciation, but the differences were not significant. We discuss two explanations for our failure to detect differences between the two clades. First, current methods for analysing species-level phylogenies may be sensitive to taxonomic and sampling artefacts. Second, lentic and lo tic clades may indeed display similar levels of species turnover despite oc cupying very different habitats at different spatial scales. More work is n eeded to investigate the effects of population level processes and spatial scale on macroevolutionary dynamics.