These results provide direct experimental support for macroevolutionary patterns of testis size evolution. Furthermore, we have shown that sperm competition selects for traits likely to be important in sexual conflicts over paternity, a result only previously demonstrated in Drosophila melanogaster.

Sperm competition theory predicts increased spermatogenic investment with increased sperm competition risk when competition is numerical. There is ample correlational evidence for this relationship in a wide range of taxa. However, as with all correlations, this does not establish cause and effect. Nevertheless, there are no published experimental studies of the evolutionary influence of sperm competition on testis size. We report here on evolutionary responses of testis size to variation in sperm competition intensity in the yellow dung fly. Experimental flies were divided across two treatments, polyandrous or monogamous, with four replicates of each. There was a rapid evolutionary response in testis size resulting from selection via sperm competition, with larger testes found when sperm competition intensity was greatest. These results provide direct experimental evidence of evolutionary change consistent with macro‐evolutionary patterns found across a wide range of taxa.

No abstract

Studies of phenotypic selection in natural populations often concentrate only on short time periods and do not quantify selection intensities. We quantified temporal and microspatial variation in the intensities of natural and sexual selection for body size in the yellow dung fly over 2 years. Female fecundity selection intensity remained approximately constant over the season with an overall mean ± SE of 0.187 ± 0.014. Selection intensity for male reproductive success, defined as eggs obtained by mating males, did not differ from zero, indicating there was no assortative mating by size. Sexual selection intensity for male mating success favouring large males was variable but overall strong in the two years (0.499 ± 0.053 and 0.510 ± 0.051). As theoretically expected for male–male competition, sexual selection intensity increased with competitor density and reached an asymptote at about 250 males per pat; it also decreased with time in spring and increased again in autumn as a function of density. Small males had the best chance of obtaining a female at very low male densities. Greater selection intensity for large size in males than females is consistent with, and might be responsible for, the observed sexual size dimorphism in this species, as males are larger. The seasonal pattern of mean male body size (smallest at the beginning and end of the season) most likely reflects mere environmental (primarily temperature) influences on phenotypic size.

The life cycles of sexually reproducing animals and flowering plants begin with male and female gametes and their fusion to form a zygote. Selection at this earliest stage is crucial for offspring quality and raises similar evolutionary issues, yet zoology and botany use dissimilar approaches. There are striking parallels in the role of prezygotic competition for sexual selection on males, cryptic female choice, sexual conflict, and against selfish genetic elements and genetic incompatibility. In both groups, understanding the evolution of sex-specific and reproductive traits will require an appreciation of the effects of prezygotic competition on fitness.

Abstract. Both female choice and male-male competition may take place during reproduction in many species. Female choice tends to be less obvious than male-male competition and consequently has received less attention from researchers. The opportunity for cryptic female choice arises after multiple insemination. Through postcopulatory processes, a female could alter the pattern of paternity among her offspring so that it does not directly reflect the different contributions of sperm made by her mates. To be able to determine if a female alters the relative sperm contributions of her mates, the behaviors and influences of the males must therefore be first taken into account. The interest of each male is to father all the offspring, and the interest of each female is to maximize paternal quality. Female yellow dung flies have complex internal reproductive tracts that may give them considerable control over the fertilization success of stored sperm from different males. In laboratory trials to date, the last male to mate has usually been most successful. In the present study, cryptic choice occurred in Scathophaga stercoraria and the pattern of choice was consistent with previously reported results. The fertilization success of a female's second mate (P2) was substantially larger if a female was kept at constant temperature and if the second male was genetically similar to her at the phosphoglucomutase (Pgm) locus. Females from the field normally have three spermathecae, but some have four. Lines were bred to have either three or four spermathecae. Flies from the different lines were crossed to generate females with similar genetic backgrounds that had either three or four spermathecae. P2 was significantly lower for high-quality females, that is, those that laid larger-than-average-clutches, with four spermathecae than for low-quality females with four spermathecae; female quality had no influence on P2 for females with three spermathecae. The results suggest that only large females may benefit from increased spermathecae number by being able to act against male interests. Females may only have three spermathecae, even though genetic variation for more is present, because selection for more spermathecae is weak.

No abstract

Sperm morphometry is extremely variable across species, but a general adaptive explanation for this diversity is lacking. As sperm must function within the female, variation in sperm form may be associated with variation in female reproductive tract morphology. We investigated this and other potential evolutionary associations between male and female reproductive characters across the Scathophagidae. Sperm length was positively associated with the length of the spermathecal (sperm store) ducts, indicating correlated evolution between the two. No association was found between sperm length and spermathecal size. However, the size of the spermathecae was positively associated with testis size indicating co‐evolution between male investment in sperm production and female sperm storage capacity. Furthermore, species with a higher degree of polyandry (larger testes) had longer spermathecal ducts. However, no associations between sperm length or length variation and testis size were found which suggests greater sperm competition sensu stricto does not select for longer sperm.