Do sexually selected weapons drive diversification?

Sexual selection is often thought to promote speciation. This expectation
is largely driven by the fact that sexually selected traits can influence
mating patterns and contribute to reproductive isolation. Indeed, some
comparative studies have shown that clades with sexually selected traits
have increased rates of speciation and diversification. However, these
studies have almost exclusively focused on one mechanism of sexual
selection: female choice. Another widespread mechanism is male-male
competition. Few empirical studies (if any) have investigated the role of
this alternative mechanism in driving diversification. Nevertheless,
recent reviews have suggested that male-male competition can increase
speciation rates. Here, we investigated whether traits associated with
precopulatory male-male competition (i.e. sexually selected weapons) have
promoted speciation and diversification in insects. We focused on three
clades with both weapons and suitable phylogenies: leaf-footed and
broad-headed bugs (Coreidae+Alydidae; ~2,850 species), stick insects and
relatives (Phasmatodea; ~3,284 species), and scarab beetles
(Scarabaeoidea; ~39,717 species). We found no evidence that weapon-bearing
lineages in these clades have higher rates of speciation or
diversification than their weaponless relatives. Thus, our results suggest
that precopulatory male-male competition may not have strong, general
effects on speciation and diversification in insects, a group encompassing
~60% of all described species.