A photosynthetic ancestry for all eukaryotes?
flies (Cyrtodiopsis dalmanni) offers a new sort of test. Sensory exploitation is the idea that ‘selection favors male traits that exploit sensory biases in females’4. These sensory biases are ‘built into the sensory modality being used and may have no intrinsic adaptive value’s, Thus, ‘the female’s sensory system defined the possible evolutionary alternatives for more attractive male traits’6. Proponents of sensory exploitation argue that biases in females’ neural machinery are evolutionarily ancient, antedating the appearance of the male traits they promote. Phylogenetic reconstructions have been used to infer the history of such ‘preexisting’ biases in fish (Xiphophorus)7,8 and frogs (Physa/aemus)4,g, but conflicting phylogenies have resulted in tenuous concIusionsl,lo. Wilkinson and Reillo’s experiment2,3 suggests a different, potentially useful approach. They selected male flies for large (L) or small (S) eye span relative to body size, and examined females’ mate choices after 13 generations. Females from L-lines preferred L males (as did unselected controls), but S-line females preferred S males. These results implied ‘a genetic correlation between female preference and a sexually selected male trait’z, an interpretation that has been disputed11 and defendedI*. Biases in females’ neural machinery that are strong enough to steer male phenotypic evolution should be difficult to alter experimentally. Yet preferences of female C. dalmanni were rapidly reversed as a by-product of selecting for S males, implying that considerable genetic variability underlies their current expression. Sensory exploitation posits that fixed female preferences drive sexual selection. Plasticity in female preferences is not consistent with this hypothesis. Comparative studies using Wilkinson and Reillo’s approach (e.g. Ref. 13) will be useful for clarifying in other species whether or not female preferences represent fixed biases imbedded in their neural machinery.