Sexual Selection and the Non-random Union of Gametes: Retesting for Assortative Mating by Fitness in Drosophila melanogaster.

While numerous theoretical population genetic models predict that mating assortatively by genetic 'quality' will enhance the efficiency of purging of deleterious mutations and/or the spread of beneficial alleles in the gene pool, empirical examples of assortative mating by quality are surprisingly rare and often inconclusive. Here, we set out to examine whether fruit flies (Drosophila melanogaster) engage in assortative mating by body-size phenotype, a composite trait strongly associated with both reproductive success and survival, and is considered a reliable indicator of natural genetic quality. Male and female flies of different body-size classes (large and small) were obtained under typical culture conditions which allows us to use standing variation of body size without involving artificial nutritional manipulation, so that their interactions and mating patterns could be measured. While flies did not exhibit assortative courtship behaviour, when patterns of offspring production were analyzed, it was found that individuals produced more offspring with partners of similar quality/body size, resulting in a positive correlation in fitness between mates. Subsequent assays of offspring fitness indicated that assortative mating produced sons and daughters that had greater or equal reproductive success than those produced from disassortative mating. Together, these results validate theoretical predictions that assortative mating sexual selection can enhance the effects of natural selection and consequently the rate of adaptive evolution.