Historical isolation facilitates species radiation by sexual selection: insights from Chorthippus grasshoppers

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 assortative mating is unrelated to the ecological
environment and primarily results from sexual selection. We address this
question using sympatric grasshopper species of the genus Chorthippus,
which have evolved strong behavioral isolation 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 shows that hybrids are absent in
sympatric localities but that all species have experienced gene flow,
confirming that reproductive isolation is strong but remains incomplete.
Demographic modelling is most consistent with a long period of geographic
isolation, followed by secondary contact and extensive introgression. Such
initial periods of geographic isolation might facilitate the association
between male signaling and female preference, permitting the coexistence
of sympatric species that are genetically, morphologically, and
ecologically similar, but otherwise behave mostly as good biological
species.