A novel cricket morph has diverged in song and wing morphology across island populations

Abstract

Abstract Divergence of sexual signals between populations can lead to speciation, yet opportunities to study the immediate aftermath of novel signal evolution are rare. The recent emergence and spread of a new mating song, purring, in Hawaiian populations of the Pacific field cricket (Teleogryllus oceanicus) allows us to investigate population divergence soon after the origin of a new signal. Male crickets produce songs with specialized wing structures to attract mates from afar (calling) and entice them to mate when found (courtship). However, in Hawaii, these songs also attract an eavesdropping parasitoid fly (Ormia ochracea) that kills singing males. The novel purring song, produced with heavily modified wing morphology, attracts female crickets but not the parasitoid fly, acting as a solution to this conflict between natural and sexual selection. We've recently observed increasing numbers of purring males across Hawaii. In this integrative field study, we investigated the distribution of purring and the proportion of purring males relative to other morphs in six populations on four islands and compared a suite of phenotypic traits (wing morphology, calling song and courtship song) that make up this novel signal across populations of purring males. We show that purring is found in varying proportions across five, and is locally dominant in four, Hawaiian populations. We also show that calling songs, courtship songs and wing morphology of purring males differ geographically. Our findings demonstrate the rapid pace of evolution in island populations and provide insights into the emergence and divergence of new sexual signals over time. Abstract What occurs immediately following the emergence of a successful new sexual signal? The fleeting nature of these evolutionary events makes them difficult to study, yet they have important implications for our understanding of speciation. Several years ago, a new male cricket morph arose in Hawaii that produced a novel sexual signal (song) using altered wing morphology. This novel ‘purring’ song is attractive to female crickets but protective against a deadly, eavesdropping parasitoid fly, serving as a new private mode of communication among crickets. In this study, we show that despite arising very recently, purring males are now found in multiple populations across Hawaii and have even reached fixation in one. Perhaps surprisingly, we also find that the wings and songs of purring males now differ geographically. The accelerated timeframe in which these changes have occurred provides new insights into how sexual signals can diverge in small island populations.