Fluctuating asymmetry and sexual selection
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Fluctuating asymmetry
Trait
Ornaments
Abstract Asymmetry has been demonstrated to play a role in signalling systems such as sexual selection and pollination, with receivers showing a preference for symmetrical signals. Large signals often have the smallest degree of asymmetry, a finding that is consistent with signal asymmetry being condition-dependent. The kind of asymmetry displayed by signals was supposed or shown to be fluctuating asymmetry, and signals revealing individual differences in the ability to stabilize developmental processes, despite a hostile developmental environment, was supposed to be the basis for the preference for symmetric signals. Recently, it has been suggested that condition-dependent signals display antisymmetry rather than fluctuating asymmetry, based on analyses of the relationship between asymmetry and mean length of the left and the right character in a few published graphs of absolute asymmetry of signals. Here I demonstrate on the basis of a much larger number of data sets, including those previously published, that the previous results are biased because of the methods used for the analyses, and that characters with condition-dependent asymmetry show fluctuating asymmetry rather than antisymmetry. In particular, frequency distributions of signed left-minus-right character values display leptokurtosis, as predicted if asymmetry distributions reflected individual differences in developmental precision, rather than platykurtosis. Platykurtosis is predicted if the traits are antisymmetric. The preponderance of leptokurtic distributions is consistent with recent modelling showing that inherent differences in the ability of individuals to control developmental processes invariably leads to leptokurtic distributions of signed left-minus-right character values.
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Developmental stability reflects the ability of individuals to cope with their environment during ontogeny given their genetic background. An inability to cope with environmental and genetic perturbations is reflected in elevated levels of fluctuating asymmetry and other measures of developmental instability. Both trait size and symmetry have been implicated as playing an important role in sexual selection, although their relative importance has never been assessed. We collected information on the relationship between success in sexual competition and size and asymmetry, respectively, to assess the relative importance of these two factors in sexual selection. Studies that allowed comparison of the relationships for the same traits' size and symmetry and success in sexual competition constituted the data, which totaled 73 samples from 33 studies of 29 species. The average sample-size weighted correlation coefficients between mating success or attractiveness and size and asymmetry, respectively, were used as measures of effect size in a meta-anatysis. Analysis was conducted on samples, studies, and species separately. We found evidence of an overall larger effect of symmetry at the species level of analysis, but similar effects at the sample or study levels. The difference in effect size for character size and character symmetry was larger for secondary sexual characters than for ordinary morphological characters at the level of analysis of samples. The results lend support to the conclusion that symmetry plays an important general role in sexual selection, especially symmetry of secondary sexual characters.
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In species where males have several ornaments for mate attraction, each ornament may coevolve with a different female preference. Alternatively, multiple ornaments may be sexually selected for because they stimulate the same, single, female preference. In the latter case, measures of preferences for different ornaments are essentially measures of the same phenotypic character and, thus, will be strongly pleiotropic, whereas no such expectation exists for multiple preferences. We selected directly up and down on two visual ornaments, orange and black coloration, in male guppies (Poecilia reticulata) in order to impose indirect selection on the female preference or preferences for these ornaments. Preferences for the two colors responded in a pattern similar to the response of the ornaments themselves. That female preferences for orange and for black in this guppy population are able to respond to selection more or less independently, suggests that they are probably two different characters in an evolutionary sense. Each of these preferences appears to be genetically correlated with the respective ornament.
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It has been postulated that levels of fluctuating asymmetry in human faces may be negatively related to components of fitness such as parasite-resistance; hence potential mates with low levels of asymmetry may appear more attractive. However, previous investigations of the relationship between asymmetry and facial attractiveness have confounded manipulations of asymmetry with facial 'averageness' and mean trait size. In this experiment we performed a manipulation that altered asymmetry within a face without altering the mean size of facial features. These faces were then rated on attractiveness. Contrary to what was predicted, faces that were made more symmetrical were perceived as being less attractive. These results do not support the hypothesis that attractiveness is related to low levels of fluctuating asymmetry. The observed positive relationship between asymmetry and facial attractiveness may be because certain facial features (including those contributing to attractiveness) in fact show directional asymmetry or antisymmetry. Our manipulations thus render naturally asymmetric features symmetrical. This may make symmetric faces less attractive because of the reduction of natural directional asymmetries, perhaps making the faces appear unemotional. The role of fluctuating asymmetries alone in assessments of facial beauty is still unknown, although this experiment suggests fluctuating asymmetry is relatively unimportant compared with directional asymmetry.
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Facial symmetry
Trait
Facial attractiveness
Antisymmetry
Physical attractiveness
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In species where males have several ornaments for mate attraction, each ornament may coevolve with a different female preference. Alternatively, multiple ornaments may be sexually selected for because they stimulate the same, single, female preference. In the latter case, measures of preferences for different ornaments are essentially measures of the same phenotypic character and, thus, will be strongly pleiotropic, whereas no such expectation exists for multiple preferences. We selected directly up and down on two visual ornaments, orange and black coloration, in male guppies (Poecilia reticulata) in order to impose indirect selection on the female preference or preferences for these ornaments. Preferences for the two colors responded in a pattern similar to the response of the ornaments themselves. That female preferences for orange and for black in this guppy population are able to respond to selection more or less independently, suggests that they are probably two different characters in an evolutionary sense. Each of these preferences appears to be genetically correlated with the respective ornament.
Ornaments
Mating preferences
Poecilia
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Fluctuating asymmetry
Trait
Ornaments
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We tested seven hypotheses regarding the mechanisms by which fluctuating asymmetry (FA) originates. We did this by analyzing data on four bilateral characters measured repeatedly during the development of individual domestic fowl. Immediately posthatching, there was substantial directional asymmetry, which rapidly decreased. We detected FA at significant levels in all characters in the majority of our measurements over the remainder of development. We also examined the effects of known environmental stressors (food and density stress) on levels of FA. At the levels we examined, changes in these stressors did not alter the degree of asymmetry we found in fowl. Time series of asymmetry for individuals did not exhibit regular oscillations, as much of the relevant literature predicts. Asymmetry levels reflected the combined effects of developmental noise, which was random in degree and direction, and feedback processes, which decreased asymmetry by altering growth rates on both sides of the body. Our findings best fit the predictions of the residual asymmetry and compensatory growth hypotheses, which suggest that levels of asymmetry reflect only recent growth history.
Fluctuating asymmetry
Fowl
Degree (music)
Stressor
Environmental stress
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Fluctuating asymmetry
Trait
Ornaments
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Fluctuating asymmetry has received considerable recent attention in evolutionary biology as these small developmental asymmetries can be related to biological fitness and, hence, could be used as a visual cue (or signal) of quality among individuals. The ability of signal receivers to detect and respond to small asymmetries is a fundamental assumption of the symmetry–signalling hypothesis, but has not been experimentally investigated. In this study I have investigated the perceptual threshold to detect and respond to paired–bar length asymmetry in a common bird, the European starling Sturnus vulgaris, by means of operant–learning techniques. The threshold indicates how large the length asymmetry must be to be reliably discriminated from symmetry; birds could not detect an asymmetry of 1.25%. In nature, many asymmetries can be smaller than 1.25%, hence this initial study suggests that caution should be used when trying to invoke symmetry–signalling in natural populations.
Sturnus
Fluctuating asymmetry
Starling
Signalling
SIGNAL (programming language)
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Ornaments
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Scramble competition
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