Intralocus sexual conflict
Throughout nature, males and females possess significantly different evolutionary interests, which are thought to be due to each sex occupying their different individual reproductive roles. The overall goal behind both sexes is reproductive success, which both male and female acquire through different techniques. For example, males do not mind dispensing their sperm into random females since their sperm are biologically less costly to produce and are found in abundance. They invest little time in the development of their offspring, and invest more energy on attaining their next mate. Male reproductive success is limited by the number of females he mates with, and not by the number of sperm that is produced. On the contrary, reproductive success in females is limited by the amount of eggs that are produced. The number of offspring created cannot exceed the number of eggs a female possesses. Unlike sperm, eggs are biologically very costly and numbered. In order for females to increase their reproductive success, they invest more time in creating high quality offspring and being choosy. Both sexes choose their mates based on favorable traits that are indicative of their mate's quality and fertility.
Natural selection leads offspring to inherit favorable traits from each parent. This will make them likely to succeed in competition of resources and reproduction. However, traits that may be optimal for the fitness of one sex may not be optimal for the other. Since many organisms have a shared set of chromosomes, organisms are restricted from individually reaching optima for their individual sex. This as a result causes sex-specific reductions in fitness, and sexual conflict between males and females.
Conflict between the two sexes commonly arises in one of two possible situations. Members of one sex can acquire characteristics that benefit their ability to successfully reproduce, while negatively impacting the fitness of the opposite sex. This process is referred to as Interlocus sexual conflict. In genetics, a locus refers to the exact location of a gene on a chromosome. This type of sexual conflict occurs at different loci in each sex. This may result in a counter adaptation in an attempt to reduce harm in the sex that experiences reduced fitness at a different locus. For example, a female may diminish detrimental consequences of being subjected to male accessory gland proteins during mating by waiting longer to re-mate, or she may develop an opposite physical adaptation of her reproductive tract Alternatively, sex-specific reductions of fitness may occur within the same locus. These specific reductions of fitness are caused by different selective pressures that exist between the sexes. They are referred to as intralocus sexual conflict. Intralocus sexual conflict could ultimately displace the opposite sex, or both, from ever reaching a phenotypic optimum.
Intralocus sexual conflict
Intralocus sexual conflict occurs because many phenotypic traits are determined by a common set of genes which are found in both male and female individuals. For example, phenotypic traits such as body size, diet, development time, longevity, and locomotory activity have been suggested to underlie intralocus sexual conflict These traits may be subjected to antagonistic patterns of selection. In intralocus sexual conflict, individuals are not able to reach their evolutionary optima independently due to limitations that have been put in place by developmental systems. As a result, this type of conflict results in a negative fitness correlation between male and female individuals that have a particular trait A very good example of this can be seen in humans, regarding the selection pressures on height that varies between sexes. In nature, a negative correlation between the height of a woman and her reproductive success has been seen, with selection favoring relatively shorter women. On the other hand, men of average height are preferred, and have higher reproductive success than men who are shorter or taller in nature. Studies have been able to produce substantial evidence that concludes that higher reproductive success is obtained by females in sibling pairs that were shorter in height than males. Contrarily, reproductive success in sibling pairs of average height was much higher in males. These findings show that intralocus sexual conflict over a physical trait, such as height, can have an effect on Darwinian fitness in the human species.
Chenowith et al. proposed a four phase model for the development of intralocus sexual conflict, in which the first phase is stabilizing selection on a trait in both sexes. Intralocus conflict then originates in the second phase when a change in physical or social conditions causes intense selection on that trait in males and/or females, and both sexes are displaced from their optimum. In the third phase, diverging selection continues on both sexes, but is attenuated. In the fourth phase, intralocus conflict is fully resolved and sexual dimorphism has occurred.
Intralocus sexual conflict diminishes the benefits of sexual selection. Examples of intralocus sexual conflict can be seen all throughout nature. In the Ibiza wall lizard (Podarcis pityusensis), intralocus sexual conflict exist over color. In this species, color is used as a signal of male fighting ability. Males that are more brightly colored are perceived as better fighters. As lizards in this species age, they become larger and more colorful. During mating seasons, males will typically compete for females and resources by fighting with each other. Males will select opponents based on the intensity of the color of their opponent's coat. Females of this species also possess brightly colored coats. This trait is detrimental for females, since being colorful makes them more conspicuous to males[clarification needed] and predators. However, in males, being colorful helps males win fights and increases their reproductive success. Another example can be seen in the features of the soay sheep (Ovis aries) horns, and the length of the serin finch's (Serinus serin) tail. Males that possess larger horns or longer tails in these species have higher success during male competition and increased reproductive success. However, these features require a great deal of energy for females to possess and do not benefit females in any significant way. In humans, males and females who appear to be more masculine in their physical appearance for their sex report to have brothers that score a higher mate value relative to their sisters. Similarly, individuals who are of normal weight and have higher levels of estradiol are positively correlated with higher mate values in women, and higher levels of testosterone are positively correlated with higher mate values in men. Individuals that are physically and hormonally more masculine tend to have brothers that are fairly more attractive than their sisters, while more feminine individuals have sisters that are more attractive than their brothers. This suggests that intralocus sexual conflict can mediate and determine the fitness of an individual. Another classic example is the human hip, where females need larger hips for childbirth as opposed to smaller hips (optimal for walking) for males. The genes that affect hip size must reach a compromise that is at neither the male optimum nor the female optimum.
There have been several hypotheses made that attempt to explain possible resolutions for intralocus sexual conflict. In one proposition, it is suggested that intralocus sexual conflict can be minimized through sex-dependent gene regulation. By doing this, genes that are negatively selected may evolve sexually dimorphic traits that encourage sex- specific optima. Sexual dimorphism is thought to be an effective resolution, since it can be made irreversible under short term selection. As a result, sexual dimorphism could pose as a resolution to intralocus sexual conflict. Another proposed hypothesis suggests that intralocus sexual conflict can be resolved through alternative splicing. In this mechanism, the gender of an organism will ultimately decide the final form of the protein that is created from a shared coding region within a set of genes. Through this posttranscriptional process, RNA that is created by a gene is spliced in various ways that allow it to ultimately join exons in a variety of ways Genomic imprinting also presents as a possible resolution for intralocus sexual conflict. In genomic imprinting, genes are marked through methylation of DNA with information of its parental origin. In order for genomic imprinting to resolve intralocus sexual conflict, parents would have to imprint their genes in sex- specific matter. For example, males could imprint their genes in a way so that sexually antagonistic alleles that benefit males are not expressed in sperm that is only X- bearing However, the resolution of intralocus sexual conflict is not easy to resolve. Intralocus sexual conflict is limited by the shared genotypes that exist between the traits of males and females. As a result, these shared genotypes often result in opposite fitness outcomes for each gender
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