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PubMed1974

Effects of population size and selection intensity on responses to disruptive selection in Drosophila melangaster.

Disruptive selection for sternopleural bristle number with opportunity for random mating was done in the four treatment combinations of two population sizes (40 pairs and 8 pairs of selected parents) and two selection intensities (1 in 40 and 1 in 2). In each generation, matings among selected parents were observed in a mating chamber, and progeny collected separately from each female parent. In the high number, high selection intensity treatment, divergence between the high and low parts ceased about generation 11. The isolation index increased rapidly to generation 3, but then fluctuated to termination of the population at generation 17. The overall isolation index was significant, indicating a real tendency to assortative mating. The failure of the isolation index to increase after generation 3 was attributed to lower average mating fitness of high males (due to inbreeding) and reduced receptivity of low females (due to a homozygous lethal gene with a large effect on sternopleural bristle number in heterozygotes). In the two low number treatments, isolation indices fluctuated from generation to generation with no obvious trends, and none of the overall isolation indices were significantly different from zero. The high number, low selection intensity treatment showed very little divergence, and one of the replicates showed, in contrast with expectation and the high number, high selection intensity treatment, a significant tendency to disassortative mating. Intense disruptive selection may lead to assortative mating.

Animals↗
PubMed2006

Lonely hearts or sex in the city? Density-dependent effects in mating systems.

Two very basic ideas in sexual selection are heavily influenced by numbers of potential mates: the evolution of anisogamy, leading to sex role differentiation, and the frequency dependence of reproductive success that tends to equalize primary sex ratios. However, being explicit about the numbers of potential mates is not typical to most evolutionary theory of sexual selection. Here, we argue that this may prevent us from finding the appropriate ecological equilibria that determine the evolutionary endpoints of selection. We review both theoretical and empirical advances on how population density may influence aspects of mating systems such as intrasexual competition, female choice or resistance, and parental care. Density can have strong effects on selective pressures, whether or not there is phenotypic plasticity in individual strategies with respect to density. Mating skew may either increase or decrease with density, which may be aided or counteracted by changes in female behaviour. Switchpoints between alternative mating strategies can be density dependent, and mate encounter rates may influence mate choice (including mutual mate choice), multiple mating, female resistance to male mating attempts, mate searching, mate guarding, parental care, and the probability of divorce. Considering density-dependent selection may be essential for understanding how populations can persist at all despite sexual conflict, but simple models seem to fail to predict the diversity of observed responses in nature. This highlights the importance of considering the interaction between mating systems and population dynamics, and we strongly encourage further work in this area.

Animals↗
PubMed1995

Mating in the monogamous male: behavioral consequences.

In monogamous mammals, males typically show selective affiliation with a single mate, high levels of paternal care, and aggression towards conspecifics to protect male and offspring. We have previously described how selective aggression and affiliation increase after mating in the male prairie vole, Microtus ochrogaster. The current studies further explored the behavioral changes that follow mating in the male of this species. The first set of experiments tested males on several behavioral measures after 24 h of either mating, social (but not sexual) exposure, or no social contact. After 24 h of mating, but not after the other two conditions, aggression and affiliation (partner preference) increased as previously reported. In addition, mated animals showed increased exploration of the open arms of a plus maze, consistent with decreased fearfulness. There were no group differences in paternal behavior (which was high in all three conditions) or analgesia (assessed by tail flick latency). To determine the minimum amount of mating necessary for the induction of aggression, males were tested in a resident-intruder paradigm after 1,6, or 24 h of mating. Although 1 h of mating was associated with a transient increase in the frequency of threats and attacks, the full spectrum of enduring aggression was observed only in the males given 24 h of mating. In a final experiment, the behavioral consequences of mating were studied in males of the closely related montane vole (Microtus montanus) which does not pair bond. Males of this nonmonogamous vole species did not show increased aggression, partner preference, or alterations in plus maze exploration following 24 h of mating. These results demonstrate the importance of prolonged mating for the induction of pair bonding in the monogamous male and they suggest that increases in aggression and affiliation are associated with decreased fearfulness in pair bonded males.

Aggression↗
PubMed2001

How important are direct fitness benefits of sexual selection?

Females may choose mates based on the expression of secondary sexual characters that signal direct, material fitness benefits or indirect, genetic fitness benefits. Genetic benefits are acquired in the generation subsequent to that in which mate choice is performed, and the maintenance of genetic variation in viability has been considered a theoretical problem. Consequently, the magnitude of indirect benefits has traditionally been considered to be small. Direct fitness benefits can be maintained without consideration of mechanisms sustaining genetic variability, and they have thus been equated with the default benefits acquired by choosy females. There is, however, still debate as to whether or not males should honestly advertise direct benefits such as their willingness to invest in parental care. We use meta-analysis to estimate the magnitude of direct fitness benefits in terms of fertility, fecundity and two measures of paternal care (feeding rate in birds, hatching rate in male guarding ectotherms) based on an extensive literature survey. The mean coefficients of determination weighted by sample size were 6.3%, 2.3%, 1.3% and 23.6%, respectively. This compares to a mean weighted coefficient of determination of 1.5% for genetic viability benefits in studies of sexual selection. Thus, for several fitness components, direct benefits are only slightly more important than indirect ones arising from female choice. Hatching rate in male guarding ectotherms was by far the most important direct fitness component, explaining almost a quarter of the variance. Our analysis also shows that male sexual advertisements do not always reliably signal direct fitness benefits.

Animals↗
PubMed2005

Correlational selection leads to genetic integration of body size and an attractive plumage trait in dark-eyed juncos.

When a trait's effect on fitness depends on its interaction with other traits, the resultant selection is correlational and may lead to the integration of functionally related traits. In relation to sexual selection, when an ornamental trait interacts with phenotypic quality to determine mating success, correlational sexual selection should generate genetic correlations between the ornament and quality, leading to the evolution of honest signals. Despite its potential importance in the evolution of signal honesty, correlational sexual selection has rarely been measured in natural populations. In the dark-eyed junco (Junco hyemalis), males with experimentally elevated values of a plumage trait (whiteness in the tail or "tail white") are more attractive to females and dominant in aggressive encounters over resources. We used restricted maximum-likelihood analysis of a long-term dataset to measure the heritability of tail white and two components of body size (wing length and tail length), as well as genetic correlations between pairs of these traits. We then used multiple regression to assess directional, quadratic, and correlational selection as they acted on tail white and body size via four components of lifetime fitness (juvenile and adult survival, mating success, and fecundity). We found a positive genetic correlation between tail white and body size (as measured by wing length), which indicates past correlational selection. Correlational selection, which was largely due to sexual selection on males, was also found to be currently acting on the same pair of traits. Larger males with whiter tails sired young with more females, most likely due to a combination of female choice, which favors males with whiter tails, and male-male competition, which favors both tail white and larger body size. To our knowledge, this is the first study to show both genetic correlations between sexually selected traits and currently acting correlational sexual selection, and we suggest that correlational sexual selection frequently may be an important mechanism for maintaining the honesty of sexual signals.

Animals↗
PubMed2007

Quantitative measure of sexual selection with respect to the operational sex ratio: a comparison of selection indices.

Despite numerous indices proposed to predict the evolution of mating systems, a unified measure of sexual selection has remained elusive. Three previous studies have compared indices of sexual selection under laboratory conditions. Here, we use a genetic study to compare the most widely used measures of sexual selection in natural populations. We explored the mating and reproductive successes of male and female bank voles, Clethrionomys glareolus, across manipulated operational sex ratios (OSRs) by genotyping all adult and pup bank voles on 13 islands using six microsatellite loci. We used Bateman's principles (Is and I and Bateman gradients) and selection coefficients (s' and beta') to evaluate, for the first time, the genetic mating system of bank voles and compared these measures with alternative indices of sexual selection (index of monopolization and Morisita's index) across the OSRs. We found that all the sexual selection indices show significant positive intercorrelations for both males and females, suggesting that Bateman's principles are an accurate and a valid measure of the mating system. The Bateman gradient, in particular, provides information over and above that of other sexual selection indices. Male bank voles show a greater potential for sexual selection than females, and Bateman gradients indicate a polygynandrous mating system. Selection coefficients reveal strong selection gradients on male bank vole plasma testosterone level rather than body size.

Animals↗
PubMed2002

Differential adult survival and phenotypic variation in a chromosomally polymorphic population of the grasshopper Sinipta dalmani.

Small size results from homozygosity for the polymorphic M4 inversion in the grasshopper Sinipta dalmani. There is directional sexual selection in males favouring the standard sequence and larger individuals. Here I report differential adult male survival in relation to body size and shape in a chromosomally polymorphic population of S. dalmani. Adult male selection was examined by scoring the same five morphometric traits in two consecutive age classes (young and aged males). Multivariate analysis showed significant differences in size between the two samples so that young males are larger for total, third (3 degrees) femur and thorax lengths while aged males show a phenotypic superiority for the lengths of third (3 degrees) tibia and tegmina. Size-adjusted scores for each trait revealed that there are significant differences between young and aged males in shape. In S. dalmani phenotypic selection associated with male mating success may be explained by means of directional selection on body size, while phenotypic selection connected with differential adult male survival may be related to both size and shape.

Analysis of Variance↗
PubMedDate not supplied

Familial versus mass selection in small populations.

We used diffusion approximations and a Markov-chain approach to investigate the consequences of familial selection on the viability of small populations both in the short and in the long term. The outcome of familial selection was compared to the case of a random mating population under mass selection. In small populations, the higher effective size, associated with familial selection, resulted in higher fitness for slightly deleterious and/or highly recessive alleles. Conversely, because familial selection leads to a lower rate of directional selection, a lower fitness was observed for more detrimental genes that are not highly recessive, and with high population sizes. However, in the long term, genetic load was almost identical for both mass and familial selection for populations of up to 200 individuals. In terms of mean time to extinction, familial selection did not have any negative effect at least for small populations (N< or =50). Overall, familial selection could be proposed for use in management programs of small populations since it increases genetic variability and short-term viability without impairing the overall persistence times.

Family↗
PubMed1981

Preferential mating in symmetric multilocus systems.

A class of multilocus models that incorporate both preferential mating and viability selection is studied. Symmetry in alleles is supposed, resulting in the phenotypes being dependent only on the location of heterozygous loci. Otherwise, an arbitrary number of loci, number of alleles per locus, and arbitrary recombination schemes, viability parameters, and preferential mating pattern are allowed. The conditions for stability of a central polymorphism, c(*), are indicated and interpreted. Mating and viability parameters enter as one combined quantity for each phenotypic class, which represents a generalized fitness. The effect on stability of c(*) of increasing the number of alleles per locus and the number of loci requires the formulation to be set in terms of frequency-dependent preference parameters.

Journal Article↗
PubMed1999

Evolutionary transition from single to multiple mating in fungus-growing ants.

Queens of leafcutter ants exhibit the highest known levels of multiple mating (up to 10 mates per queen) among ants. Multiple mating may have been selected to increase genetic diversity among nestmate workers, which is hypothesized to be critical in social systems with large, long-lived colonies under severe pressure of pathogens. Advanced fungus-growing (leafcutter) ants have large numbers (104-106 workers) and long-lived colonies, whereas basal genera in the attine tribe have small (< 200 workers) colonies with probably substantially shorter lifespans. Basal attines are therefore expected to have lower queen mating frequencies, similar to those found in most other ants. We tested this prediction by analysing queen mating frequency and colony kin structure in three basal attine species: Myrmicocrypta ednaella, Apterostigma collare and Cyphomyrmex longiscapus. Microsatellite marker analyses revealed that queens in all three species were single mated, and that worker-to-worker relatedness in these basal attine species is very close to 0.75, the value expected under exclusively single mating. Fungus growing per se has therefore not selected for multiple queen mating. Instead, the advanced and highly productive social structure of the higher attine ants, which is fully dependent on the rearing of an ancient clonal fungus, may have necessitated high genetic diversity among nestmate workers. This is not the case in the lower attines, which rear fungi that were more recently derived from free-living fungal populations.

Animals↗
PubMed1995

Study of immunological dysfunction in periparturient Holstein cattle selected for high and average milk production.

Data from twenty assays of traits associated with innate and adaptive immunity were evaluated from 137 periparturient Holstein cows. These cows had been selected through planned matings for four different levels of milk production (high and average pounds of milk, and high and average pounds of milk fat plus protein). For up to seven generations, the genetic lines were produced by mating females of each line to sires of corresponding merit. With the exceptions of neutrophil ingestion of Staphylococcus aureus and directed migration, all assays measuring neutrophil functions were depressed beginning 2 to 3 weeks before calving through 3 weeks after calving. Serum concentrations of immunoglobulin G1 decreased while those of immunoglobulin G2 increased around calving time. Serum complement and conglutinin concentrations decreased before calving and reached a minimum around calving time. Cows selected for high milk production (pounds of milk and pounds of milk fat plus proteins) had significantly higher (P < 0.10) numbers of circulating neutrophils and mononuclear cells, had higher (P < 0.10) neutrophil resting chemiluminescence and higher (P < 0.10) neutrophil directed migration than cows with average production potentials. There were significant (P < 0.001) sire progeny group differences for most traits associated with the immune system that we tested. These results can be considered encouraging, in that selection for high milk yield did not produce unfavorable correlated responses in the functional capacity of immune function traits, and that there is sufficient genetic variation in these immunological traits among sires of high genetic merit for milk production to potentially improve the immunocompetence of periparturient cows through planned mating experiments.

Animals↗
PubMed1996

Conditional Manipulation of Sex Ratios by Ant Workers: A Test of Kin Selection Theory

Variable queen mating frequencies provide a unique opportunity to study the resolution of worker-queen conflict over sex ratio in social Hymenoptera, because the conflict is maximal in colonies headed by a singly mated queen and is weak or nonexistent in colonies headed by a multiply mated queen. In the wood ant Formica exsecta, workers in colonies with a singly mated queen, but not those in colonies with a multiply mated queen, altered the sex ratio of queen-laid eggs by eliminating males to preferentially raise queens. By this conditional response to queen mating frequency, workers enhance their inclusive fitness.

Journal Article↗
PubMed1995

Mutations affecting donor preference during mating type interconversion in Saccharomyces cerevisiae.

Homothallic strains of Saccharomyces cerevisiae can convert mating type from a to alpha or alpha to a as often as every generation, by replacing genetic information specifying one mating type at the expressor locus, MAT, with information specifying the opposite mating type. The cryptic mating type information that is copied and inserted at MAT is contained in either of two loci, HML or HMR. The particular locus selected as donor during mating type interconversion is regulated by the allele expressed at MAT. MATa cells usually select HML, and MAT alpha cells usually select HMR, a process referred to as donor preference. To identify factors required for donor preference, we isolated and characterized a number of mutants that frequently selected the nonpreferred donor locus during mating type interconversion. Many of these mutants were found to harbor chromosome rearrangements or mutations at MAT or HML that interfered with the switching process. However, one mutant carried a recessive allele of CHL1, a gene previously shown to be required for efficient chromosome segregation during mitosis. Homothallic strains of yeast containing a null allele of CHL1 exhibited almost random selection of the donor locus in a MATa background but were normal in their ability to select HMR in a MAT alpha background. Our results indicate that Chl1p participates in the process of donor selection and are consistent with a model in which Chl1p helps establish an intrinsic bias in donor preference.

Alleles↗
PubMed2006

No evidence of current sexual selection on sexually dimorphic traits in a bird with high variance in mating success.

Sexual dimorphism, particularly in ornamental traits, is likely to have arisen by sexual selection. Most empirical and theoretical studies of sexual dimorphism assume that ongoing sexual selection also maintains the dimorphism. Over four seasons, I measured the sexual selection acting on three sexually dimorphic attributes (epaulet size, body size, and the blackness of the body plumage) of male red-winged blackbirds and found no consistent directional or stabilizing selection on any of them. Correlational selection was also negligible. I used path analysis to explore potential relationships in more detail but found no direct or indirect effects of male traits on either within- or extrapair success. Males who were resident on the marsh for more years had higher within-pair success, primarily because they spent more of the season on their territory. Experimental manipulations of epaulet size and color and the extent of nonblack feathers in the black body plumage had no detectable effect on the number of within-pair mates, paternity, or the number of extrapair offspring sired in nearby territories. These results combine with data from other studies of red-winged blackbirds to suggest that, despite high variation in male mating success and hence a strong opportunity for sexual selection, several morphological attributes that differ between the sexes and vary among males are not under current sexual selection. The possible explanations for why add complexity to our understanding of how sexual selection operates.

Animals↗
PubMed2005

The Maynard Smith model of sympatric speciation.

The paper entitled "Sympatric speciation," which was published by John Maynard Smith in 1966, initiated the development of mathematical models aiming to identify the conditions for sympatric speciation. A part of that paper was devoted to a specific two-locus, two-allele model of sympatric speciation in a population occupying a two-niche system. Maynard Smith provided some initial numerical results on this model. Later, Dickinson and Antonovics (1973) and Caisse and Antonovics (1978) performed more extensive numerical studies on the model. Here, I report analytical results on the haploid version of the Maynard Smith model. I show how the conditions for sympatric and parapatric speciation and the levels of resulting genetic divergence and reproductive isolation are affected by the strength of disruptive selection and nonrandom mating, recombination rate, and the rates of male and female dispersal between the niches.

Animals↗