Sexual selection and the random union of gametes: testing for a correlation in fitness between mates in Drosophila melanogaster.

Both males and females vary in fitness. While high-fitness males typically have greater siring success, it is not clear whether these males sire an equal fraction of offspring from all females or a disproportionately large fraction with high-fitness females. The latter nonrandom reproductive pattern can arise as the result of sexual selection and creates a positive correlation in fitness between mates. Such a correlation, if it reflects a positive genetic correlation between mates with respect to fitness, increases the efficiency of selection, reducing mutation load and speeding adaptation. While there is evidence from many taxa that assortative mating for fitness may occur, these studies typically focus on observed matings rather than realized reproductive output. Here, we examine assortative mating for fitness in Drosophila melanogaster, first in the context of virgin matings and then using a measure of realized reproduction that incorporates remating and postcopulatory processes. We find evidence for positive assortative mating among virgins but no evidence of assortative mating using the more complete measure of reproduction.