Résumé
A key challenge for biologists is to document and explain global patterns
of diversification in a wide range of environments. Here, we explore
patterns of continental-scale diversification in a groundwater
species-rich clade, the superfamily Aselloidea (Pancrustacea: Isopoda).
Our analyses supported a constant diversification rate during most of the
course of Aselloidea evolution, until 4–15 Ma when diversification rates
started to decrease. This constant accumulation of lineages challenges the
view that groundwater species diversification in temperate regions might
have been primarily driven by major changes in physical environment
leading to the extinction of surface populations and subsequent
synchronous isolation of multiple groundwater populations. Rather than
acting synchronously over broad geographic regions, factors causing
extinction of surface populations and subsequent reproductive isolation of
groundwater populations may act in a local and asynchronous manner,
thereby resulting in a constant speciation rate over time. Our phylogeny
also revealed several cases of parapatric distributions among closely
related surface-water and groundwater species suggesting that species
diversification could also arise from a process of disruptive selection
along the surface-subterranean environmental gradient. Our results call
for re-evaluating the spatial scale and timing of factors causing
diversification events in groundwater.