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9 Mate Choice and Sexual Selection: What Have We Learned Since Darwin?--Adam G. Jones and Nicholas L. Ratterman
Pages 169-190

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From page 169...
... And second, what factors determine the strength of mate choice (or intensity of sexual selection) in each sex?
From page 170...
... Darwin believed that sexual selection played a major role in the evolution of humans and the divergence among distinct human populations, so he felt a lengthy description of sexual selection was necessary. indeed, the bulk of the book concerns sexual selection, but many of Darwin's insights regarding sexual selection appear in his chapters on humans.
From page 171...
... . our sojourn through sexual selection literature leads to the identification of at least 2 major triumphs of precopulatory sexual selection research since Darwin.
From page 172...
... in fact, Darwin correctly realized that even in species characterized by the "law of battle," mate choice often is important. As Darwin stated: The female could in most cases escape, if wooed by a male that did not please or excite her; and when pursued, as so incessantly occurs, by several males, she would often have the opportunity, whilst they were fighting together, of escaping with, or at least temporarily pairing with, some one male.
From page 173...
... hence, Darwin clearly understood that female preferences existed, but he never compellingly explained why such preferences would evolve. The flavor of Darwin's argument for female choice may represent one of the largest shortcomings of his treatment of sexual selection because it gave the impression that animals would need a human-like sense of aesthetics for sexual selection to operate.
From page 174...
... it can be argued that it took almost 100 years for the study of sexual selection to overcome this erroneous view of mating preferences. The First Major Triumph of Modern Sexual Selection Research The study of sexual selection entered its modern era during the latter half of the 20th century when scientists identified the evolution of female choice as a legitimate topic in its own right by expanding the ideas laid out much earlier by Fisher (1915, 1930)
From page 175...
... 263) Unfortunately, Darwin did not elaborate much on this line of reasoning, and he never provided a convincing explanation for the evolution of a preference for ornaments or other sexually selected characters not directly involved in territory defense or parental care.
From page 176...
... TABle 9.1 some empirical examples supporting various Models of Mate Preference evolution Mechanism of Preference evolution some sample organisms references Direct-benefits models Bushcrickets Gwynne (1984) resource acquisition Protection elephant seals, dung flies Galimberti et al.
From page 177...
... our reading of the literature indicates that direct-benefits models enjoy excellent empirical support and are mostly not controversial, so we will treat them no further here. Models of Indirect Benefits in some species, the males appear to provide nothing to the females but sperm, yet they have elaborate ornaments for which females show preferences (Taylor and Williams, 1982; reynolds and Gross, 1990; Kirkpatrick and ryan, 1991; Kokko et al., 2003)
From page 178...
... it is also worth noting that some amount of a genetic correlation between ornament and preference will occur in any system in which mate choice operates, so a Fisherian process could act in concert with almost any other model of mate-choice evolution. Model 2: The Condition-Dependent Indicator Model The condition-dependent indicator model is the most widely used of the "good genes" models (Maynard smith, 1991; Andersson, 1994; Mead and Arnold, 2004)
From page 179...
... The major remaining challenges for this model are to explain what could maintain a genetic correlation between male viability and the ornament trait and to find additional empirical examples of this process. Other Models of Mate-Choice Evolution in addition to the direct- and indirect-benefits models of mate choice, several other models have been proposed.
From page 180...
... Maybe this process is operating in the background in all systems characterized by mate choice or maybe it comes into play episodically when conditions are especially favorable for its operation. on the other hand, depending on the evolution and maintenance of genetic correlations between traits and preferences, the possibility remains that the Fisherian process explains very little with respect to the evolution of female preferences.
From page 181...
... every model of choice evolution is affected by assumptions regarding the costs of choosing, but insufficient empirical data exist from this challenging area of inquiry to properly parameterize the models. Perhaps we need a new concept called "the environmental potential for mate choice" in which costs of choice are considered in light of reproductive ecology.
From page 182...
... some of the differences among species are almost certainly due to natural selection on the traits or differences in sex-limited inheritance, but in some cases, especially in the case of secondary sexual characteristics, the differences among lineages are due to dissimilar intensities of sexual selection. in other words, while keeping in mind that observable sexual dimorphism is not an entirely reliable guide to the strength of sexual selection, a complete explanation for morphological differences among taxa requires consideration of factors affecting sexual selection in populations of organisms.
From page 183...
... in addition, Darwin clearly had a good intuitive understanding of the concept of the operational sex ratio (which is the ratio of males ready to mate to receptive females in the population) and the implications of mating patterns.
From page 184...
... . hence, a nonzero mating differential will result in a nonzero selection differential only if the Bateman gradient is positive.
From page 185...
... This figure is not meant to be the final word on the subject, as we believe there is a great need for additional work. Factors Affecting Mating Differentials The question here is how ecological factors can either enhance or decrease the likelihood of a correlation between sexually selected trait values and mating success.
From page 186...
... if mate searching is efficient, very strong mating differentials may result, whereas constrained searching may impose limitations. in addition, the operational sex ratio (emlen and oring, 1977)
From page 187...
... and others that have yet to be appreciated, that affect mating differentials. Factors Affecting Bateman Gradients As noted above, a mating differential will result in a selection differential on a trait only if high mating success results in high relative fitness, and the Bateman gradient describes this conversion (Arnold and Duvall, 1994; Andersson and iwasa, 1996; Jones et al., 2000; Jones, 2009)
From page 188...
... Ratterman fundamental Bateman relationship graph certainly could affect the Bateman gradient. The most obvious such factor is the operational sex ratio (emlen and oring, 1977)
From page 189...
... Two of the greatest triumphs included the proliferation of models explaining the evolution of female preferences and quantitative approaches to the measurement of selection differentials. These advances provide clear, plausible mechanisms for the evolution of female choice and allow us to begin to address why sexual selection varies among species.
From page 190...
... Ratterman Despite the triumphs of modern sexual selection research, there are still many topics that need to be addressed. For example, some models of the evolution of mate choice enjoy only limited empirical support, and for the most part we are not sure which model explains the majority of choice evolution within or between systems.


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