EVOLUTION OF ANIMAL GENITALIA - MORPHOLOGICAL CORRELATES OF FITNESS COMPONENTS IN A WATER STRIDER

Rapid divergence of male genitalia is one of the most general evolutio nary trends in animals with internal fertilization, but the mechanisms of genital evolution are poorly understood. The current study represe nts the first comprehensive attempt to test the main hypotheses that h ave been suggested to account for genital evolution (the lock-and-key, sexual selection and pleiotropy hypotheses) with intraspecific data. We measure multivariate phenotypic selection in a water strider specie s, by relating five different components of fitness (mating frequency, fecundity, egg hatching rate, offspring survival rate and offspring g rowth rate) to a suite of genital and non-genital morphological traits (in total 48). Body size had a series of direct effects in both sexes . Large size in females was positively related to both fecundity and e gg hatching rate. There was positive sexual selection for large size i n males (mating frequency), which to some extent was offset by a reduc ed number of eggs laid by females mated to large males. Male genitalic morphology influenced male mating frequency, but the detected directi onal selection on genitalia was due to indirect selection on phenotypi cally correlated non-intromittent traits. Further, we found no assorta tive mating between male intromittent genitalia and female morphology. Neither did we find any indications of male genitalia conveying infor mation of male genetic quality. Several new insights can be gained fro m our study. Most importantly, our results are in stark disagreement w ith the long standing lock-and-key hypothesis of genital evolution, as well as with certain models of sexual selection. Our results are, how ever, in agreement with other models of sexual selection as well as wi th the pleiotropy hypothesis of genital evolution. Fluctuating asymmet ry of bilaterally symmetrical traits, genital as well as non-genital, had few effects on fitness. Females with low fluctuating asymmetry in leg length produced offspring with a higher survival rate, a pattern m ost probably caused by direct phenotypic maternal effects. We also dis cuss the relevance of our results to sexual conflict over mating, and the evolution of sexual traits by coevolutionary arms races between th e sexes.