Changes in morphology are often thought to be linked to changes in species diversification,

which is expected to leave a signal of early burst (EB) in phenotypic traits.

However, such signal is rarely recovered in empirical phylogenies, even for groups

with well-known adaptive radiation. Using a comprehensive phylogenetic approach

in Dytiscidae, which harbours ~4,300 species with as much as 50-fold variation in

body size among them, we ask whether pattern of species diversification correlates

with morphological evolution. Additionally, we test whether the large variation in

body size is linked to habitat preference and whether the latter influences species

turnover. We found, in sharp contrast to most animal groups, that Dytiscidae body

size evolution follows an early-burst model with subsequent high phylogenetic conservatism.

However, we found no evidence for associated shifts in species diversification,

which point to an uncoupled evolution of morphology and species

diversification. We recovered the ancestral habitat of Dytiscidae as lentic (standing

water), with many transitions to lotic habitat (running water) that are concomitant

to a decrease in body size. Finally, we found no evidence for difference in net diversification

rates between habitats nor difference in turnover in lentic and lotic species.

This result, together with recent findings in dragonflies, contrasts with some

theoretical expectations of the habitat stability hypothesis. Thus, a thorough

reassessment of the impact of dispersal, gene flow and range size on the speciation

process is needed to fully encompass the evolutionary consequences of the lentic–

lotic divide for freshwater fauna.