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The ecological mechanisms that contribute to evolution

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October 1st Assignment

https://en.wikipedia.org/wiki/Talk:Isodar

Edits to an existing Wikipedia Page https://en.wikipedia.org/wiki/Talk:Sexual_selection

Final Draft Starts Here

Sexual selection involves competition within one sex for members of the opposite sex, and variety in choice by one sex for members of the opposite sex. Sexual selection selects for traits within each sex based on parental investment or ethological factors. The effects of sexual selection are : deviations from the phenotypic optimum, sexual dimorphism of characteristics, speciation, extinction, or parental investment determination. These effects resulting from sexual selection lead to the phenotypic evolution in different populations. The concept of sexual selection contributes to evolutionary biology by shaping the characteristics exhibited by one sex or the other based on not only the appeal of favorable traits, but the choice made by one sex on what traits are favorable or 'fit'. In addition, it affects the appearance of new species and the disappearance of existing species.

The sexual selection of traits can be based on many factors, but the first factor discussed will be how parental investment can affect how sexual selection operates. As explained,sexual selection entails one sex being choosier for their mates and one sex will normally have a trait that is being selected for. If one sex invests more in terms of becoming a parent, then the other sex will be the ones participating in intraspecific competition. For males that are less parentally invested in their offspring, males usually have adaptations that allow them to reproduce in a short period time with the tradeoff of dying faster. For males that are more parentally invested in their offspring, males usually have adaptations that allow for them to live longer, but they will then experience issues in terms of reproductive success. The issues for reproductive success in this case involve cuckoldry. (Trivers, R. 1972) Traditionally the sex with less parental investment will compete for the sex that has more parental investment. The results of the study in the Trivers article indicate that sexual selection can act on both sexes, but the concept of parental investment is the determining factor of which sex chooses their mate and which sex competes for their mate.

Parental investment can also be a result of sexual selection rather than governing it in terms of determination of which sex plays which role. A study performed on the Tanganyikan cichilds indicated that parental care patterns were not affected by female investment or the way the young are cared for. In addition, the results show that sexual selection can dictate patterns of parental investment. The species which experienced more intense sexual selection displayed a different pattern of parental care that entailed mainly females becoming the primary caretakers rather than sharing the duties. In contrast, the species which were subject to more moderate sexual selection displayed the pattern of care entailing both parents as equal contributors to the offspring's care (biparental care). (Gonzalez-Voyer, A., 2008) This study is a strong indicator of parental investment being not only a component deciding how sexual selection will occur, as previously indicated by Trivers, but also a result of sexual selection as seen in the Tanganyikan cichilds.

Sexual selection operates through another factor besides parental investment, ethological factors. The ethological factors are the behaviors displayed by a certain species. A study observing Drosophila melanogaster attempts to determine components of sexual selection. Ethological factors were determined to have a significant effect on the occurrence of sexual selection. The study found that ethological factors, however, differ in the strength of its effects based on whether it is subject to intraspecific or interspecific competition. For intraspecific competition, ethological factors are less important in terms of sexual selection because it is masked by the sexual vigor of the fly species. The vigor is related to the amount of females the male can inseminate and the period of time the male can court. However, at the interspecific level, ethological or behavioral factors become the primary factor in sexual selection among the Drosophila and the sexual vigor becomes secondary in importance. (Faugeres, A., et al., 1971) The study concluded that sexual selection is controlled by the behavior/ethological factors and these factors are predominant at the interspecific level of sexual selection.

After discussing some factors that dictate how sexual selection operate, the effects of sexual selection is the next step in understanding its importance in species. One of the effects of sexual selection is the sexual dimorphism of sexual characteristics between two sexes. Dimorphism of these characters entails the very distinct difference in the phenotype of the sexes indicated by difference in size, behavior and ornamentation of male versus female. Dimorphism is key to the in regards to sexual selection because of the emphasis on a male trait that is selected for. The male trait is shown phenotypically, but the difference in the phenotype of the sexes is not the only factor. Female choice is also indicated because in unstable phases for a population, both, dimorphism of secondary sexual characters of males and female choice are shown to increase exponentially. As a result, the rate of revolution is very rapid. (Lande. R, 1981) However, by having exponential growth, there will come a point when the selection for the extremes must end. The selection for extremes ends because the extreme males and discriminating females will have trouble finding a mate: this is known as counterselection in which the once favorable trait is selected against. The models presented in Lande's study show how sexual selection can clearly impact the strength of dimorphism in characters between the sexes, which can eventally lead to rapid evolution of extreme traits as well as the evolution of female choice for these extremes.

In the Arnold and Wade article, the authors analyze the data on bullfrogs in order to determine the magnitude of sexual selection in comparison to other forms of selection that could have occured in the selection of bull frog sizes. This study complements the previously discussed study in regards to sexual dimorphism of characteristcs. From the data, the results indicate that the mean, or average phenotypic characteristics are shifted away from due mainly to sexual selection. For the bullfrogs, the sexual selection that occurs on the mean body sizes is clearly shown by the shift in mean body size that is by almost two-thirds of a standard deviation. The authors assert that there is still a method in terms of determining the contribution of the different forces of selection by dividing them up, which can also give a clearer picture in terms of life stages. In different life stages, populations or species may experience different forms of selection. The bullfrog data indicated that during one particular mating season, is when the selection for malebullfrog sizes was mainly due to the selective force of sexual selection with smaller forces of natural selection at work. This article ties into the effects of sexual selection as indicated from the Lande article by demonstrating that the opposite sex does indeed undergo changes in characters such as size in order to be selected as a mate. The favored character in this case is size for male bullfrogs and the choosier sex will most likely be the female bullfrogs.

Another effect of sexual selection on populations is the deviation from what is the optimal phenotype for both sexes as a result of sexual selection. Based on the conceptual models in Lande's study, he asserts that sexual selection acts in a way that becomes less favorable in terms of obtaining the best or most 'fit' phenotype for both sexes. The sexual selection that will act on a particular character can negatively impact a population. The negative impact entails sexual selection acting upon a character that is already under weak natural selection, which can cause the average or mean phenotype to deviate from the optimum phenotype. (Lande, R., 1980) This,in turn, results in decreases in the overall fitness of a population. As a result, by having sexual selection act on a character in this sitation, the overall fitness of a population can decrease and the probability of extinction will increase. The evolution of the mean phenotypes in this case does not have favorable results in terms of fitness and demonstrates the effects of sexual selection on a population's general characteristics which is applicable to both sexes. The result on the phenotypic evolution on each sex differs in that one sex will have a more extreme phenotype that demonstrates which trait is selected for by the other sex, while the choosier sex will become increasingly more discriminatory by only mating with the extreme phenotypes.

Another effect of sexual selection is the speciation that can occur as a result of strong sexual selection. Rapid speciation can occur as well as normal speciation that involves complete reproductive isolation. Models of speciation and its progression are explored in Lande's article and it explains that there is an unstable phase which shows the exponential increase of male traits being selected for and female choice. One of the effects resulting form this is the creation of a new species because of the sexual isolation that can occur when only the extreme phenotypes are mating with the extremely selective females for a particular trait. This can in turn result in the phenotypic divergence of a population from its relatives and these effects are indirectly a result of sexual selection in action. The author discusses and indicates that sexual selection can in the manner mentioned above, achieve rapid speciation because of the reproductive isolation that results from sexual selection. (Lande. R, 1981) The sexual selection is supplemented by the concept of sexual dimorphism which allows for the males to display their extreme ornamentation. Another study that complements the concept of speciation is seen in Svedin's study of hybrid flycatchers. In the study, they found a large amount of selection against male hybrids. The hybrids experience a reduction in their fitness by one half of the regular flycatchers and this results from the hybrid disadvantage in competing for a mate. Because the hybrid males are under strong selection in a disadvantageous way, sexual selection plays an important force for the possibility of the hybrids becoming reproductively isolated. "This role of sexual selection in causing hybrid unfitness has important implications for speciation." (Svedin, N., et al., 2008) The concept of sexual selection is made clear in its effects in speciation and the articles complement each other well by extending how sexual selection can cause the speciation that is prevalent in evolutionary biology.

The final effect of sexual selection discussed is extinction. When the sexual selection becomes extremely intense, there is an increase probability that extinction of a species can occur as seen in Morrow's study of birds. The results from the study indicate that species who are subject to an intense or high amount of post-mating sexual selection, have a larger threat in terms of extinction. The study, however, indicates that pre-mating sexual selection does not have a significant effect on species extinction. While the study determines that sexual selection, post-mating, has a higher chance of subjecting a species to extinction, the reasoning for this has not been defined. The researchers have narrowed the possibilities to either the absolute population fitness being lower due to or the overall selection load is higher due to sexual selection. (Morrow, E.H., et al., 2003) The link between intensity of sexual selection and possible extinction is also touched upon in the Lande article from 1980 that was previously discussed in regards to deviation from optimal phenotype. In his study, he also indicates that intense sexual selection can indirectly decrease the overall fitness of a population and therefore increase the probabilty for extinction, a concept that is the focus of Morrow's study of sexual selection in birds.

An important concept that is utilized in many of these studies is the runaway process. The runaway process involves male traits and female preferences increasing geometrically with time until finally checked by counterselection. This process can create a new species by sexual isolation and phenotypic diververgence of a population from its closest relatives. It is a key concept in many of the articles because it indicates how intense sexual selection is because of the exponential rate at which it can occur for a population. The intensity of sexual selection leads to the discussed effects of deviation from an optimal phenotype and sexual dimorphism. The deviation and sexual dimorphism in turn, can result in speciation or extinction if not checked by counterselection.

Another concept that is central to sexual selection is parental investment. Parental investment entails the amount of care a parent will spend in their offspring and it differs between species as well as which sex will become more parentally invested. It affects how sexual selection will occur by deciding which sex choose their mate and which sex will compete. However, it can also be a result of sexual selection by deciding how much parental investment will come from each sex based on the intensity of sexual selection.

All of the research and studies discussed plays upon the 2 different aspecets of sexual selection: how it operates and the eventual effects of it upon a popultion. The studies all tie in to the idea that sexual selection has a large impact on the phenotypic evolution of a population. Sexual selection will operate based on parental investment and ethological behaviors, while the direct effects will cause sexual dimorphism and deviations from an optimal phentype. In addition to the phenotypic evolution as seen in the direct effects, sexual selection is an extremely powerful concept in evolutionary biology because of its large indirect effect on speciation and extinction.

References:

Arnold, S.J. Et al. (July 1984) On the Measurement of Natural and Sexual Selection: Applications. Evolution, Volume 38, Issue 4, pp. 720-734

Faugeres, A., et al. (June 1971) The Components of Sexual Selection. Evolution, Volume 25, No. 2, pp. 265-275

Gonzalez-Voyer, A., et al. (August 2008) Sexual Selection Determines Parental Care patterns in Cichild Fishes. Evolution, Volume 62, No. 8., pp. 2015-2026

Lande, R. (March 1980) Sexual Dimorphism, Sexual Selection and Adaptation in Polygenic Characters. Evolution., Volume 34, Issue 2, pp. 292-305

Lande, R. (June 1981) Models of speciation by sexual selection on polygenic traits. Proc. Natl. Acad. Sci., Volume 78, Issue 6, pp. 3721-3725

Morrow, E.H., et al. (September 2003) Sexual Selection and the Risk of Extinction in Birds. Proceedings: Biological Sciences, Volume 270, No. 1526, pp.1793-1799

Svedin, N., et al. (March 2008) Natural and Sexual Selection against Hybrid Flycatchers. Proceedings: Biological Sciences, Volume 275, No. 1635, pp. 735-744

Trivers, R. (1972). Parental investment and sexual selection. Aldine Publishing Company, Chicago, 42 p.