Body size evolution as a catalyst for diversification of copulation strategies and its spillover effects on genital evolution and phylogenetic divergence in terrestrial arthropods

Abstract To explore how ecological diversity contributes to the establishment of ecologically similar species with sexual divergence, this review focuses on body size—an environmentally influenced trait that determines male insemination capacity. It also summarizes the results of 49 studies on how body size variation impacts copulatory performance, genital evolution, and speciation rate in 34 taxonomic groups of terrestrial arthropods with promiscuous mating systems. Larger males consistently had larger ejaculate production. In more than half of the studies, increased male body size was associated with increased ejaculate volume, shortened copulation duration, and increased copulation frequency. Moreover, large body sizes often contributed to paternity acquisition under sperm competition. Among the four taxa for which the relationship between body size evolution and genital evolution was examined while considering phylogeny, two beetle taxa showed exaggerated genitalia exclusively in some lineages with larger body sizes and high speciation rates. In one of the taxa, a positive relationship between body size enlargement and genital exaggeration has been confirmed, probably due to increased sperm competition. In contrast, no such positive relationship was observed in the remaining two taxa (millipedes and spiders), possibly due to male–female interactions such as sexual conflict. This review demonstrates that increased body size resulting from environmental adaptation should enhance sperm competition and has the potential to initiate speciation through genital divergence.