Sexual selection fails to promote adaptation to a new environment

Selection can be divided into sexual and nonsexual components. Some work finds that a component of sexual selection, adaptive female selection for good genes, can promote nonsexual fitness. Less studied is the benefit from sexual selection in toto, that is, when intra- and intersexual selection are both present and able to affect females directly and indirectly. Here an upper bound for the net benefit of sexual selection is estimated for Drosophila melanogaster. Replicate populations were allowed to adapt to low-grade thermal stress, with or with out the operation of sexual selection. Because proteins and lipids are highly sensitive to temperature, low-grade thermal stress will select broadly across the genome for alternative alleles. Such broad, directional selection for thermal tolerance should increase the measurable benefits of sexual selection far beyond that available under stabilizing selection. Sexual selection was removed by enforced monogamy without mate choice and retained by enforced polyandry (four males per female). After 36 generations of thermal stress exposure, there was substantial adaptation to the new environment (the net reproductive rate increased six standard deviations relative to thermal controls). However, sexual selection did not affect the rate of adaptation. Therefore, adaptive female selection for thermal tolerance either was insignificant or negated by other aspects of sexual selection, for example, male-induced female harm, which has been shown to diminish under monogamy. This experiment employed two parameters that reduced the opportunity for divergence in such harm: a truncated intersexual interaction period and strong directional selection for thermal tolerance. No divergence in male-induced harm was observed.