Calculations for quantifying the potential for sexual selection remain controversial. Many indices have been promoted in the literature, but each has unique sets of advantages and disadvantages.
Using marbled salamanders, I evaluated the performance of several measures by manipulating intensity of sexual selection in experimental breeding replicates of varying operational sex ratio.
Theory predicts that sexual selection among males will be higher when sex ratio is male-biased and lower when female-biased. I used microsatellite data to assign hatchling parentage, estimate adult fitness, and calculate several indices of inequality for quantifying sexual selection.
In the present study, although variation in both reproductive and mating success was high when quantified using opportunities for selection, it was only significantly greater than random expectations for reproductive success. This study provides further empirical support for the continued use of opportunity for selection in sexual selection studies. Although sexual selection was first discussed over a century ago Darwin, and has been a major topic in the evolution literature for decades, there is still no consensus on how we should quantify its action.
Most often, people have used a calculation related to formal sexual selection theory opportunity for selection, I that grew out of what came to be known as Bateman's principles Arnold,as first discussed when Bateman published his descriptions of the mating system in Drosophila fruit flies Bateman, His ideas were based on the importance of fitness variance in determining the strength of sexual selection.
Specifically, Bateman's principles suggest that the sex that has higher variance in mating success number of mates and reproductive success number of progeny experiences more sexual selection. I has come under attack over the last few decades for three main reasons. First, it does not incorporate random variation that is expected to occur even in the absence of selection.
For example, Sutherland showed that Bateman's data can be explained by random chance alone and may not have reflected the action of sexual selection.
Sexual selection theory evaluation further showed that differences in fitness variance between the sexes may result from simple randomness paired with sex differences in the time required for mating i. When mean fitness increases, I tends to decrease such that its expected value is the reciprocal of the mean.
This is expected for any random variable with a Poisson distribution for which mean and variance are equal. Indeed, the upper bound of I is equal to the number of individuals in the group. Several have suggested that we use other less biased measures to maximize the robustness
Sexual selection theory evaluation these types of comparisons. Many indices of inequality have been derived, used in various studies, and promoted as the best options Ruzzante et al.
Generally, the new indices with the most support deal with these problems better than I. All express fitness variation relative to random expectations, some are restricted to values below one to ameliorate group size dependence, and mean dependence may not be as severe as in I.
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Each of the proposed indices, however, has unique properties, and
Sexual selection theory evaluation vary considerably in their ability to resolve the problems for which I has been attacked. There is no clear reason to choose any particular measure as a replacement for Iand compelling arguments for its continued use have been made Jones et al.
Most notably, it is the only measure that has a formal tie to mathematical sexual selection theory. Attempts to evaluate some of these measures have offered contrasting conclusions Kokko et al.
More work is needed to assess the strengths and weaknesses of the competing measures, and broad characterization of sexual selection in nature will not be possible until evolutionary biologists resolve this disagreement.
Collecting additional empirical data to assess genetic parentage in mating system studies can help rectify the controversy. Although Jones et al. Subsequently, Mills et al.
None of these comparative "Sexual selection theory evaluation" included the binomial skew index Nonacs, in their analyses, even though it has been promoted as a good measure for all inequalities and performs well in simulation studies Nonacs, For example, when sex ratio is male-biased, sexual selection is expected to be higher among males than when it is female-biased. Thus, in principle, we can control the potential for sexual selection experimentally by manipulating OSR, which allows the use
Sexual selection theory evaluation empirical data to compare the performance of different measures for quantifying the potential for sexual selection.
I used this approach with marbled salamanders Ambystoma opacuma terrestrially breeding species. This study aimed to contribute to the discussion about how we should quantify sexual selection by comparing the performance of several popular measures. Five indices of inequality were evaluated by varying the OSR of breeding groups of marbled salamanders in cattle tanks. Polymorphic microsatellite markers were used to assign parentage of hatchlings via strict exclusion, fitness of adults was estimated, and the following measures were calculated for comparison: None of the previous evaluative studies included all five of these measures, and one was unable to look at reproductive success.
This study is intended to offer further empirical evaluation of the indices, the need for which is emphasized by the controversy about the best way to quantify the action of sexual selection. Marbled salamanders Ambystoma opacum breed terrestrially in temporarily dry wetland basins of the eastern USA.
Reproductive adults migrate from surrounding terrestrial habitats to breeding sites during warm, rainy nights in autumn. Mating often occurs in mass aggregations of courting males and receptive females.
Courtship consists of mutual nudging and stereotyped movements that culminate in the male depositing a spermatophore on the ground. After mating, females oviposit beneath logs, vegetation, and other debris and attend their egg clutches during embryonic development. When seasonal rains inundate nest sites, hatchlings emerge and develop in aquatic habitat until metamorphosis.
Because males contribute only gametes to reproduction, sexual selection is expected to be higher among males than females. Galvanized steel cattle tanks circular, 1. Because marbled salamanders nest in terrestrial habitat, the tanks contained soil, logs, transplanted vegetation, cover boards, and leaf litter.
Tanks were partially
Sexual selection theory evaluation into the soil at the natural breeding site of the animals. After adding adult salamanders to the tanks on 22 Octoberscreen covers were placed over them to exclude predators. Animals were held in the laboratory for no more than 10 days before the beginning of the experiment. Tanks received either eight Sexual selection theory evaluation treatment, mean SVL: Because of limitations related to the other experiment and lack of resources, this design did not control for density and group size.
The salamanders were allowed to mate and nest over several weeks, and eggs were collected from the tanks in late November by removing nesting cover. Once the tanks had been excavated completely so that all nests were found, egg clutches were submerged in well water to induce hatching.
The number of hatchlings produced by each clutch was counted and a sample of them was preserved in pure ethanol for subsequent microsatellite DNA genotyping. After parentage assignment, each hatchling's genotype was manually compared with those of the adults in the breeding group. Individuals with potential scoring errors, identified when an allele was present only once in a hatchling sample or when there was a single discrepancy between an offspring and a putative parent, were rescored.
The four loci were sufficient to allow strict exclusion of all but the true parents in each case, except when females had mated before the experiment and produced offspring with unsampled males. "Sexual selection theory evaluation" female salamanders did not nest seven in the male-biased and three in the female-biased treatment and appeared gravid at the end of the experiment.
The analyses were performed with and without these non-nesters included, and most of the conclusions were not altered. For all analyses except the Bateman gradients, the results are reported when these females are included with zero fitness. Because not all females use the tanks for nesting, even when many males are present D. Therefore, these non-nesters were excluded from the gradient analysis. However, some of these females mated with males that were present in the tanks, and resulting offspring were incorporated into estimates of mating and reproductive success of males.
Mating success was estimated as the number of genetic mates or the number of adults with which each salamander produced progeny. Because entire clutches could not be assayed and matings that did not result in hatchlings necessarily went undetected, estimates of mating success are conservative.
To estimate reproductive success, the proportion of each clutch's hatchlings that were sired by competing males was multiplied by the total number of hatchlings produced by that clutch. Opportunity for selection I was calculated as the variance in fitness divided by the square of mean fitness. Standard analysis of variance techniques in GLM procedure were employed to test for mean differences between treatments and sexes in the potential for sexual selection, when of interest.
Finally, analyses of covariance in GLM were employed to test for differences in Bateman gradients between the sexes and treatments. Males had more mates when the sex ratio was female-biased than when it was male-biased mean of 1. A greater proportion of males than females failed to produce offspring, despite the inclusion of non-nesting females.
Although this analysis provided some evidence that sexual selection is likely to be more important for males than females, it did not necessarily show that variance in male mating success was significantly greater
Sexual selection theory evaluation random expectations.
Sexual selection theory evaluation affected the potential for sexual selection among males Fig. All other indices yielded higher measures in female-biased treatment than in the male-biased treatment for either mating success, reproductive success, or both Fig. Solid bars represent the male-biased sex ratio treatment; open bars represent the female-biased treatment. Calculations were made using estimates of both mating success and reproductive success with five competing indices.
Numbers above each pair of bars are the one-tailed P -values from simple statistical tests for differences between treatments. There was no statistically detectable effect of sex ratio treatment on sexual selection among females.
When sexual selection was measured with I pfitness variance among males was significantly greater than expected to occur by random chance alone for reproductive success, but not for mating success i. Calculations were made using estimates of both mating success and reproductive success in both sexes and treatments with five competing indices for four means per index.
Numbers above each pair of bars are one-tailed P -values from simple statistical tests for differences between the sexes in each sex ratio treatment. Females that did not nest are included.
Females that did not nest were excluded. This study provides further empirical evaluation of the performance of five indices of inequality used by evolutionary biologists to quantify the potential for sexual selection. It is meant to add to "Sexual selection theory evaluation" very small body of literature that uses experimental manipulations of mating systems to assess the properties of
Sexual selection theory evaluation indices.
Much more of this type of work is needed, and this study contributes to the discussion by providing more data from a new mating system and evaluating five popular indices at the same time. In addition, my data demonstrate the need for testing fitness variation against random expectations in certain contexts, something that has not been discussed by recent advocates of opportunity for selection Jones et al.
Sexual selection was
Sexual selection theory evaluation among males than females, regardless of which index was used, and Bateman gradients corroborated this finding.
These results likely reflect true differences in the potential for sexual selection, and not just discrepancies
Sexual selection theory evaluation group size. First, although the male-biased treatment had more males than the female-biased treatment, our numbers are strikingly similar to those reported by Jones et al.
Second, when all males and females are considered together, so that there are two groups of similar size to compare 50 males, 44 femalesthe values of I reproductive success: Finally, as will be argued later, paired increases in I and group size only reflect real differences in fitness variance.
Differences in total density probably did not account for the observed discrepancy in the potential for sexual selection. Second, overall densities in both sex ratio treatments were necessarily high in my experiment 7. The potential density effects are likely related to increased encounter rates but, at these high densities, every salamander in both treatments likely encountered every other salamander during nights of activity.
Using marbled salamanders, I evaluated the performance of several Theory predicts that sexual selection among males will be higher when sex ratio is. Recent claims that sexual selection theory is fundamentally flawed are . evolutionary responses based on assessment of sexual selection is.
In this work, Darwin fleshed...
PDF | The application of sexual selection theory to human behavior has indicators is that the cues used in such assessments will tend to be exaggerated over.
Is talking about sex with the opposite gender always classed as flirting? Darwin proposed an explicitly aesthetic theory of sexual selection in which he Sensory evaluation includes the possibility that display traits do not encode. We do not consciously (or even unconsciously) evaluate those we meet in terms of their ability to help us procreate. We may feel a strong sexual attraction that..
Sexual selection theory evaluation
Calculations for quantifying the potential for sexual selection remain controversial.
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