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Why is sexual reproduction so common in nature?

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Introduction

Why is sexual reproduction so common in nature? One of the greatest challenges for evolutionary biology is explaining the widespread occurrence of sexual reproduction, and the associated process of genetic recombination. Sexual reproduction involves one individual combining half its DNA with half of DNA of another individual, so that the offspring is only half genetic copy of each parent. However, in asexual reproduction, the offspring are genetic copies of the parent. Thus, sexual reproduction poses an evolutionary problem because it seems to be half as efficient a method of reproducing as asexual reproduction. Asexual females can potentially produce twice as many daughters as sexual females, so that the ratio of asexual to sexual females should initially double each generation, resulting in the 'two-fold cost of sex'.? In addition to this 50% cost and the dilution of the individual's genome, sexual reproduction also presents other disadvantages in comparison to asexual reproduction. First and foremost there is the cost of recombination - favourable gene combinations that have increased in frequency under the action of natural selection are broken up. Secondly, the process of sexual reproduction requires a significant cellular-mechanical cost as sex requires meiosis, syngamy and karyogamy. A great amount of time is taken up by these three processes alone, and far exceeds the time required for two mitotic divisions. Also, these processes are unnecessarily complicated if reproduction is sole objective. Asexual spores and meiosis-bypassing apomixis appears much more efficient. ...read more.

Middle

The second of the two modern day models ignores the effect of deleterious mutations and concentrates on external environmental change. This model suggests that sex accelerates adaptation to a changing environment by creating new gene combinations. The problem is to work out how environments could possibly change fast enough, ie: at a rate that makes sex beneficial every generation. Currently, the most interesting and promising hypothesis - the Red Queen - suggests that coevolution between hosts and parasites may generate environmental change at a rate that renders sex advantageous in the long term (the 'environment' for the parasite is the host's resistance mechanism, and the 'environment' for the host is the parasite's penetration mechanism). Usually parasites are assumed to provide the antagonistic driving force in this co-evolutionary dance, though host immune systems may laso do so. The dance is a consequence of time lagged selection by co-evolving parasites against common host genotypes, leading ultimately to sustained oscillations in host and gene frequencies. Substantial evidence has been provided to support the Red Queen Hypothesis, which is why it stands as one of the more convincing theories explaining the evolution of sex. For example, Baer et al. wrote a paper on how experimental variation in polyandry affects parasite loads and fitness in a bumble bee. Through artificially inseminating queens of a bumble bee with sperm of either high or low genetic diversity, they tested the hypothesis that genetic diversity among a female's offspring may offer some protection from parasitism. ...read more.

Conclusion

The alternative combination is that the environmental RQ hypothesis aids the MD process. The problem with the MD hypothesis is that high rates of deleterious mutations are required, often greater than one mutation/genome/generation. Most of the models used to describe the MD process often assume infinite populations often assume infinite populations and consider populations at equilibrium in mutation-selection balance, bypassing the fundamental dynamics required to reach this situation. The RQ hypothesis can be manipulated to solve such dilemmas, as it produces frequency-dependent selection and so slows down the spread of asexual clones, allowing more time for mutation-selection balance to attained. West et al. then go on to argue that the RQ hypothesis reduces the fitness advantage of asexuals, reducing the number of deleterious mutations that are needed to reduce the asexual fitness below that of sexual individuals. Furthermore, it may even increase the rate at which an asexual line accumulates mutations (through stochastic process of Muller's ratchet), leading to an increased reduction in fitness and therefore allowing mutation-selection balance to be reached more efficiently. The idea put forward by West et al. is important in attempting to explain the problem that although deleterious mutations and parasite-host coevolution are reasonable theories, it still has not been conclusively shown that either really explains how sex is maintained in nature. However, further tests still need to be carried out on the pluralistic approach, meaning that sex still persists as a puzzle for evolutionary biologists. ...read more.

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Response to the question

Very well done report.Response to the question done very well. Introduction is good, as is the main body of text which examines a range of different theories and the main evidence to go with it. Conclusion should be a lot ...

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Response to the question

Very well done report.Response to the question done very well. Introduction is good, as is the main body of text which examines a range of different theories and the main evidence to go with it. Conclusion should be a lot more in depth but it is adequate for this level. The response of the client is very clear throughout and well analysed.

Level of analysis

Technical terms expressed for this level are very advanced compared to what I would expect. Also, analysis and thought processes are a lot more in depth. Also shows examples of looking at a wide range of sources to reach their conclusions which are presented in a very appropriate straight forward analytical way. The only criticism is that the conclusion should be more stand alone and the different reasons for the trains of thought that he examined weighed up and a more appropriate conclusion and the reasons for it reached. Could also include graphs and scientific research data from reports.

Quality of writing

Very high level of grammar, punctuation and spelling.


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Reviewed by skatealexia 11/03/2012

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