The handicap hypothesis of sexual selection predicts that sexual ornaments have evolved heightened condition-dependent expression. The prediction has only recently been subject to experimental investigation. Many of the experiments are of limited value as they: (i) fail to compare condition dependence in sexual ornaments with suitable non-sexual trait controls; (ii) do not adequately account for body size variation; and (iii) typically consider no stress and extreme stress manipulations rather than a range of stresses similar to those experienced in nature. There is also a dearth of experimental studies investigating the genetic basis of condition dependence. Despite the common claim that sexual ornaments are conditiondependent, the unexpected conclusion from our literature review is that there is little support from welldesigned experiments.

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Handicap models of sexual selection predict that male sexual ornaments have strong condition-dependent expression and this allows females to evaluate male genetic quality. A number of previous experiments have demonstrated heightened condition-dependence of sexual ornaments in response to environmental stress. Here we show that genetic variation underlies the response to environmental stress (variable food quality) of a sexual ornament (male eye span) in the stalk-eyed fly Cyrtodiopsis dalmanni. Some male genotypes develop large eye span under all conditions, whereas other genotypes progressively reduce eye span as conditions deteriorate. Several non-sexual traits (female eye span, male and female wing length) also show genetic variation in condition-dependent expression, but their genetic response is entirely explained by scaling with body size. In contrast, the male sexual ornament still reveals genetic variation in the response to environmental stress after accounting for differences in body size. These results strongly support the hypothesis that female mate choice yields genetic benefits for offspring.

We used the stalk-eyed fly Cyrtodiopsis dalmanni to examine predictions made by condition-dependent handicap models of sexual selection. Condition was experimentally varied by manipulation of larval food availability. Cyrtodiopsis dalmanni is a highly dimorphic species exhibiting strong sexual selection, and the male sexual ornament (exaggerated eyespan) showed strong condition-dependent expression relative to the homologous trait in females and nonsexual traits. Male eyespan also showed a great increase in standardized variance under stress, unlike nonsexual traits. The inflated variance of the male ornament was primarily attributable to condition-dependent (but body-size-independent) increase in variance. Thus, evaluation of male eyespan allows females to gain additional information about male condition over and above that given by body size. These findings accord well with condition-dependent handicap models of sexual selection.

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Aging may be a consequence of mutation accumulation or of negative pleiotropic correlations between performance late and earlier in the lifespan. This study used artificial selection on flies derived from two different base stocks to produce "young" and "old" lines, propagated by breeding from young and old adults respectively. Virgin and mated adults of both sexes from the "old" lines lived longer than "young" line flies. "Young" and "old" mated females did not differ in fecundity or fertility early in the lifespan, but "old" line females had higher fecundity and fertility late in life. The results therefore suggested either that the response to selection had revealed the effect of mutation accumulation, or that pleiotropy involving characters other than early fecundity must have been involved. Development time from egg to adult was longer in the "old" lines. Competition of selected line larvae from one base stock against mutant marked larvae from the same base stock revealed that, at a wide range of larval densities, "old" line larvae showed lower survival rates than "young" line larvae. Thorax length and wet weight were significantly greater in the "old" line flies from one base stock. The results may imply that the selection regime in the "old" lines favored extended growth during development to produce a more durable adult soma, despite the cost in increased larval mortality and delayed reproduction, because the potential reproductive benefits later in life were increased. However, the differences between larvae from "old" and "young" lines could also be attributable to density differences, and this possibility needs systematic investigation.