This apparently occurs because of the beetle’s genetic architecture, which means evolving larger mandibles results in the correlated evolution of masculinized females (even though females never develop mandibles). Previous work has shown that males with larger mandibles sire daughters with lower fecundity, and that directly selecting for increased (or decreased) mandible size results in decreased (or increased) female fitness (LRS) 40, 42. Females lack this exaggerated character completely 40, 43. The enormously enlarged male mandibles are used in male–male fights, and males with larger mandibles have higher fighting and mating success 39, 40, 41, 42. Here, we use the broad-horned flour beetle Gnatocerus cornutus to directly test for effects of predation on the evolution of a male sexually selected character (their mandibles) and female lifetime reproductive success (LRS). And again, controlled experimental tests for evolutionary responses to predation are usually not undertaken. This is especially true when intersexual genetic correlations link sexually selected male characters with female fitness, because selection on one sex can affect the other through these correlations 37, 38. Unfortunately, without exploring how predation affects both sexes, we are unlikely to fully understand how predation affects sexual trait evolution 10. Nonetheless, while there is ample evidence that predation selects on both females and males, the joint action of selection on the sexes is frequently investigated independently. Costs can accrue via delayed development, slower growth or postponed reproduction 33, 34, 35, 36. Resultant anti-predator behaviors may reduce foraging efficiency and reproductive activity, and thus be costly to females (reviewed in the ref. For example, egg-carrying females can be slower 30, 31 and suffer higher predation rates 29, 32. 26, 27).įemale reproductive success can also be impacted by predation 28, 29.
ASREML R VERIFICATION
Nonetheless, while there is ample evidence that predation selects against sexual trait enhancement, there is limited direct experimental verification that this selection causes evolutionary responses in these traits (but see e.g. 21) and males frequently reduce their sexual signaling in response to predation risk, which can result in decreased mating success when risk is high 22, 23, 24, 25. More generally, predation appears to reverse the evolution of extreme sexually selected phenotypes (reviewed in the ref. This is particularly well documented in orthopterans and frogs 12, 13, 14, 15, 16, 17, 18, and this form of natural selection is probably responsible for the loss of cricket sexual signals on two Hawaiian islands 19, 20. For example, sexual signals are conspicuous to potential mates but may also attract predators and parasitoid 11. One potentially important source of natural selection that could affect the evolution of sexually selected traits is predation 1, and many studies have shown predation can seemingly oppose the exaggeration of male sexual characters.
While theory is clear on the joint effects of natural and sexual selection on sexual trait evolution, explicit experimental tests of theoretical predictions are required to fully understand sexual trait evolution 10. They also demonstrate how natural selection can oppose sexual selection as trait values move beyond their naturally selected optima (reviewed in ref. Lande’s 5 and Kirkpatrick’s 6 models of sexual selection via the Fisher 7 process-the null models of intersexual selection 8-clearly shows how this can occur. Sexual selection typically acts more strongly on males and is responsible for the evolution of a vast array of exaggerated characters that enhance male sexual fitness components 1, 2, 3, 4. Our findings support fundamental theory, but also reveal unforseen outcomes-the indirect effect on females-when natural selection targets sex-limited sexually selected characters. Predation solely on females has no effects. We find that populations subjected to male-specific predation evolve smaller sexually selected mandibles and this indirectly increases female fitness, seemingly through intersexual genetic correlations we document. Here we use experimental evolution of replicate populations of broad-horned flour beetles to test for effects of sex-specific predation on an exaggerated sexually selected male trait (the mandibles), while also testing for effects on female lifetime reproductive success.
Empirical evidence supports this theory, but to our knowledge, there have been no experimental evolution studies directly testing this logic, and little examination of possible associated effects on female fitness. Theory shows how sexual selection can exaggerate male traits beyond naturally selected optima and also how natural selection can ultimately halt trait elaboration.
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