Sexual Selection Theory by Charles Darwin Research Paper

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Introduction

Sexual selection theory proposed by Charles Darwin stating that the frequency of traits can increase or decrease depending on the attractiveness of the bearer, has biologists today distinguish between “male to male combat” where males usually fight, “mate choice” where females usually choose male mates, and “mate coercion” forced mating.

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Consequently, traits selected for by male combat are called weapons, and traits selected by female mate choice are called ornaments. Lately, attention has been given to cryptic female choice, a phenomenon in internally fertilizing animals such as mammals and birds, where a female may simply dispose of a male’s sperm without his knowledge as Eberhard (1996) proposed. Likewise, sperm competition is the equivalent in male-to-male combat.

It has been generally accepted that the exact effect of sexual selection depends on the sex ratio usually slightly biased in favor of the “limiting” sex generally females. In addition, male-to-male combat is also classified as intrasexual competition, while mate choice and mate coercion are also known as intersexual competition. Females usually mate with males having external ornaments or exaggerated features of morphology. It was proposed that these may arise due to an arbitrary female preference for some aspect of male morphology initially increased by genetic drift, creating selection for males with the appropriate ornament known as the sexy son hypothesis. Alternatively, the good genes hypothesis proposes that in order to enable males to develop great ornaments, they simply show off greater disease resistance or a more efficient metabolism, features that also benefit females.

This paper shall try to review sexual selection in beetles for future research topics using information from the primary literature, specifically biological journals.

Discussion

Generally, sexual selection is believed to be a potent force in the evolution of morphology in sexually reproducing species where large size in a trait is favored by sexual selection the trait often exhibits positive allometry. With the premise that mating behavior in whirligig beetles consists of males attempting to grasp reluctant females using enlarged protarsi or pretarsal pads, Fairn et al (2007) used allometry and a mating experiment to investigate sexual selection pressures on accessory glands, intromittent genitalia (aedeagus), and pretarsal pads in males of the whirligig beetle Dineutus nigrior Roberts. The study was able to establish that accessory gland size exhibited positive allometry and males with larger accessory glands were more likely to copulate suggesting that larger size in this trait is favored by sexual selection. In addition, it was observed that males with larger accessory glands attempted to copulate more often but did not exhibit fewer failed mating attempts before copulating. The study suggested that the increased probability of mating in males with large accessory glands is due to higher mating attempt frequency and not to increased ability to overcome female resistance. Likewise, they were able to point that the length of the aedeagus exhibited negative allometry, and males with a longer aedeagus did not have increased mating success. This was consistent with stabilizing selection that favors an intermediate size in this trait. In addition, the allometric slope of the pretarsal pad did not differ from isometry, and males with larger pretarsal pads did not have increased mating success. Consequently, it was suggested that larger pretarsal pads are not favored by sexual selection (Fairn et al, 2007).

While it is widely assumed that male sperm competitiveness evolves adaptively, recent studies found a cytoplasmic genetic component to phenotypic variation in some sperm traits presumed important in sperm competition. Maternally transmitted, cytoplasmic genes cannot respond to selection on sperm and it was proposed that this constraint may affect the scope in which sperm competitiveness can evolve adaptively. By examining nuclear and cytoplasmic genetic contributions to sperm competitiveness using populations of Callosobruchus maculatus carrying orthogonal combinations of nuclear and cytoplasmic lineages, Dowling et al (2007) were able to examine genetic contributions to female remating. They found that sperm competitiveness and remating are primarily encoded by nuclear genes particularly, a male’s sperm competitiveness phenotype was contingent on an interaction between the competing male genotypes. Likewise, cytoplasmic effects were detected on remating but not sperm competitiveness, suggesting that cytoplasmic genes do not generally play a profound evolutionary role in sperm competition (Dowling et al, 2007).

An interesting finding on the few behaviors for infanticide has also been a recent focus on other studies. While biologists accept infanticide as adaptive, there has been disagreement in many cases over the causes of infanticide of which a study of burying beetles investigated two causes of infanticide, resource competition, and sexual selection. The study had the premise that when the potentially infanticidal intruder was of a different species, only resource competition was important. Likewise, when the intruder was of the same species but of the opposite sex there was a possibility of sexual selection as well. While in sexually selected infanticide, the intruder will kill the resident’s offspring and then pair with the resident to produce a replacement brood, in cases where both resource competition and sexual selection were operating in burying beetles, infanticide was more common. It was then proposed that the ability to produce a replacement brood with the intruder reduces the motivation to protect the young and leads to a higher incidence of infanticide (Trumbo, 2006).

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It was suggested that radioactivity may have both phenotypic and genetic effects through its disruption of physiological processes and mutations. In a study of the size and asymmetry of secondary sexual and ordinary morphological characters of stag beetles (Lucanus Cervus) in two areas in Ukraine: near Chernobyl, where levels of radiation are high, and in a control area with low background radiation, the developmental instability of morphological characters was estimated from the degree of fluctuating asymmetry using the restricted maximum likelihood parameter estimation (REML) method to partition measurement error from asymmetry. Likewise, the degree of asymmetry estimated from unsigned differences in the size of the right and left secondary sexual character was similar to estimates based on the REML method. It was found that beetles from the contaminated area had a significantly elevated level of fluctuating asymmetry in the secondary sexual character compared with animals from the control area. Male stag beetles found with a female had significantly lower asymmetry than males found alone. In addition, while mated males did not differ in asymmetry between areas, unmated males from Chernobyl were much more asymmetric than unmated males from the control area. These provide evidence for radiation disrupting developmental homeostasis and thereby affecting the mating status of free-living beetles (Møller, 2002).

Conclusion

In view of the foregoing, I conclude that varying degrees of factors and causes may affect sexual selection in beetles and that interesting and more specific areas of study may prove more variants.

In this view, I would like to propose a possible link between the physical environment’s temperature and the sexual selection process in beetles using generally acceptable opposing temperature scales.

References

Dowling, D. K. U. Friberg, & G. Arnqvist (2007). “A comparison of nuclear and cytoplasmic genetic effects on sperm competitiveness and female remating in a seed beetle.” Journal of Evolutionary Biology, Volume 20, Issue 6, Page 2113-2125, 2007

Eberhard, WG. (1996). Sexual selection by cryptic female choice. Princeton, Princeton University Press.

Fairn, Evan R., Albrecht I. Schulte-Hostedde & Yves Alarie. (2007). “Sexual Selection on Accessory Glands, Genitalia and Protarsal Pads in the Whirligig Beetle Dineutus nigrior Roberts (Coleoptera: Gyrinidae).” Ethology, 113 (3) p 257-266

Fricke, Claudia and Göran Arnqvist (2007) “RAPID ADAPTATION TO A NOVEL HOST IN A SEED BEETLE (CALLOSOBRUCHUS MACULATUS): THE ROLE OF SEXUAL SELECTION.” Evolution, 61 (2) pp. 440-454

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Møller, Anders Pape (2002). “Developmental Instability and Sexual Selection in Stag Beetles from Chernobyl and a Control Area.” Ethology, 108 (3), Page 193-204, 2002

Trumbo, Stephen (2006). “Infanticide, sexual selection and task specialization in a biparental burying beetle.” Department of Ecology and Evolutionary Biology, University of Connecticut. From www.animalbehavior.org

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