Theoretical and empirical studies have shown that species radiations are
facilitated when a trait under divergent natural selection is also
involved in sexual selection. It is yet unclear how quick and effective
radiations are where sexual selection is unrelated to the ecological
environment. We address this question using grasshopper species of the
genus Chorthippus, which have evolved strong assortative mating while
lacking noticeable eco-morphological divergence. Mitochondrial genomes
suggest that the radiation is relatively recent, dating to the
mid-Pleistocene, which leads to extensive incomplete lineage sorting
throughout the mitochondrial and the nuclear genomes. Nuclear data show
extremely low genomic differentiation among species, yet hybrids are
absent in sympatric localities. Demographic analyses shed some light
into these seemingly contradictory patterns. The estimated demographic
model shows a long period of geographic isolation, followed by secondary
contact and extensive introgression. This suggests that an initial
period of geographic isolation might favor the coupling of male
signaling and female preference, which currently maintains species
boundaries in the face of long-term gene flow. More generally, these
results suggest that sexual selection can lead to radiations without a
primary role of divergent natural selection, resulting in cryptic
species that are genetically, morphologically and ecologically similar,
but otherwise behave mostly as good biological species.