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FINAL PAPER

“Why Sexual Reproduction has Evolved and Continued to Persist in Many Higher Order Species”

The ability to produce offspring and pass on genes to new generations is the main focus for organisms throughout their lifetime. Reproduction allows for organisms to create these offspring that carry their genes and thus continue the lineage. However, the modes of reproduction are diverse. The forms of reproduction utilized throughout nature are divided into the categories of asexual and sexual reproduction. The major distinction between the two is how the genetic information is passed onto the offspring. Asexual reproduction involves the direct transfer of genetic information from the parent to the offspring. The offspring are genetic clones of the single parent. Sexual reproduction involves the fusion of two gametes, either from one or two parents, where the genetic information given to the offspring is a mixture of both. While both forms of reproduction are still used today by varying species, asexual reproduction appears to be less prevalent in more evolved species. The question still persists as to why higher order species mostly utilize sexual reproduction to further their lineage. Multiple hypotheses exist and have been studied to explain why sexual reproduction has become more common in higher order species compared to asexual reproduction. While there are many hypotheses for why the evolution of sexual reproduction has occurred, many of the reasons appear to intertwine and support one another (Howard and Lively 1997).

Asexual reproduction has brought about multiple advantages to many groups of organisms. A major benefit is the reduced amount of energy necessary to produce the next generation of offspring. Organisms partaking in asexual reproduction are not subject to meiosis in order to produce new offspring thus using less energy at a cellular level. At a more macroscopic level, asexual reproduction does not require mating between individuals. The lack of mating allows for individuals to avoid the physical energy exerted in locating a mate as well as the energy used for mating rituals or appealing characteristics preferred during sexual selection. The limited amount of time needed to produce an offspring by asexual reproduction is significantly less than that of sexual reproduction. Without the use of meiosis, the process proceeds to completion at a quicker rate. The absence of mating allows for reproduction to occur faster for similar reasons mentioned above. However, with numerous advantages, many higher order organisms continue to reproduce sexually to maintain and further their linage.

Sexually reproducing organisms have taken advantage of the unfavorable qualities of asexual reproduction. Genetic variation and recombination of genes are the most commonly discussed advantages of sexual reproduction. Organisms participating in sex mate with another individual in order to produce offspring. In doing so, meiosis in eukaryotes occurs which allows for the alignment of homologous chromosomes from the two parental organisms. The aligning of the chromosomes leads to a step within meiosis termed recombination. This allows for a rearrangement of DNA from the two parents to produce new variations of alleles that can be given to the offspring. Genetic recombination allows for a wide array of genotypic diversity throughout a population as well as between siblings or other relatives. Genetic diversity appears to be most beneficial when the organism experiences changing environments. The genetic variation present in a population or species will allow selection to show preference towards a specific group’s phenotype and eventually its genotype (Crow 1992). These organisms with the beneficial genotype will survive and continue to reproduce allowing for the genotype and alleles to have a chance of being passed onto the offspring resulting in continued adaptation to the certain environment (Colegrave 2002). In asexual reproduction, the only way to cause variation within the DNA is for a mutation to occur. Mutations in general occur at a fairly slow rate within many higher order organisms. This being said, the phenomenon to acquire the correct mutation needed for survival in a different environment may not occur prompt enough to save the organisms from death.

While DNA is able to recombine to modify alleles, DNA is also susceptible to mutations within the sequence that can affect an organism in a negative manner. Asexual organisms do not have the ability to recombine their genetic information to form new and differing alleles. Once a mutation occurs in the DNA or other genetic carrying sequence, there is no way for the mutation to be removed from the population until another mutation occurs that ultimately deletes the primary mutation. This is rare among organisms. Hermann Joseph Muller introduced the idea that mutations build up in asexual reproducing organisms. Muller described this occurrence by comparing the mutations that accumulate as a ratchet. Each mutation that arises in asexually reproducing organisms turns the ratchet once. The ratchet is unable to be rotated backwards, only forwards. The next mutation that occurs turns the ratchet once more. Additional mutations in a population continually turn the ratchet and the mutations, mostly deleterious, continually accumulate without recombination (Muller 1964). These mutations are passed onto the next generation because the offspring are exact genetic clones of their parent. The genetic load of organisms and their populations will increase due to the addition of multiple deleterious mutations and decrease the overall reproductive success and fitness. A study done at the University of Maryland showed the effects of Muller’s ratchet on asexual viruses. The accumulation of harmful mutations by the ratchet displayed a decrease in fitness of different RNA virus lineages (Chao 1990). For sexually reproducing populations, mutations in the DNA are more likely to be removed due to recombination in the process of meiosis. The offspring are also not direct genetic clones of a single parent. The alleles from both parents contribute to the offspring. This creates the ability to mask a mutation in the form of heterozygotes. Selection can also work in removing mutations from a sexual population. The lessened amounts of harmful mutations within an organism can lead to increased reproductive success. Natural selection will select for the reduced number of deleterious mutations. Many believe that this ability to evade the accumulation of harmful and possibly lethal mutations produces a substantial advantage for sexually reproducing populations.

Alexey Kondroshov contributed to the notion that mutations occur in both asexual and sexual reproducing organisms. His deterministic mutational hypothesis suggests that in sexually reproducing organisms there will be some containing a small number of deleterious mutations while others may experience a larger number of mutations. As discussed in Muller’s ratchet, Kondroshov also agreed that a buildup of harmful mutations would lead to decreased fitness within organisms and possible extinction for the species (Kondrashov 1988). Due to the ability to recombine genetic material in meiosis, different genotypes and phenotypes are produced throughout the population, which contain varying numbers of mutations. Natural selection is able to control the levels of mutations in organisms reproducing sexually. Organisms containing larger amounts of deleterious mutations and therefore a lower fitness are selected against. Lower numbers of mutations and higher fitness organisms are selected for. This appears to be a safeguard for sexually reproducing organisms to limit the amount of mutations accumulated in their genome as well as a way to increase the fitness of the population. This generalization also holds true for asexual reproducing individuals. As discussed with Muller’s ratchet, asexual organisms will continually pass down their exact genetic information. Therefore, the accumulation of mutations from the parent will be passed down to the offspring. The number of mutations continues to increase while decreasing the fitness. Natural selection will select against the accumulation of multiple harmful mutations within asexual or sexual populations. The difference between the two modes of reproducing shows that while some sexually reproducing organisms will be selected against, there will still be others that will thrive. Asexually reproducing organisms have a lesser chance for any of the population to survive since they are exact clones of each other. While many mutations are a harmful to the organisms, predators and parasites also prove to endanger the lives of organisms in their habitats. Organisms are commonly fighting with predators and other detrimental species to maintain their health and stay alive in order to reproduce. This constant battle between organisms produces an evolutionary arms race. In order for a population to prosper, it must evolve to counteract and diminish the previous evolutionary maneuver made by its opponent (Van Valen 1974). Humans are accustomed to the arms race between the human host and different types of bacteria and viruses. Humans introduce new antibiotics into the system to work against the previous evolutionary step made by the bacteria or virus. Both continue to evolve usually without an evident winner. The Red Queen hypothesis gives support to why sexually reproducing, higher order organisms continue to be prominent today. Within the book Through the Looking-Glass, the queen states to Alice “it takes all the running you can do, to keep in the same place” (Carroll 1871). The basic idea of the hypothesis is that populations must keep evolving in order to solely exist similarly to what the queen states within the book. In order to evolve and compete with the opponent, species usually change their genetic information to counteract how the opponent population has evolved previously. Sexually reproducing organisms are able to create genetic diversity as stated earlier by ways of recombination of genes. Recombination can produce unique organisms into a population very quickly. These unique organisms initially evade the opponent since the opponent has not had time to evolve and adapt to that specific genotype. This unique phenotype will be selected for and become more abundant within the population. Over time as a certain allele changes from unique to common, new changes will have to ensue to produce another variation that can evolve. Asexual organisms in comparison evolve slowly because mutations are needed in order to produce genetic diversity. These organisms may not evolve in adequate enough time against their predator to secure survival. A study completed in August 2014 illustrates this argument successfully. The study looked at both asexual and sexual reproducing snails over a period of time. Ultimately it was shown that sexual reproducing snails were less likely to be infected by a parasitic worm than asexual reproducing snails due to their ability to effectively co-evolve with the parasite (Vergara et. al. 2014). As a survival mechanism, sexual reproduction appears to be most beneficial for continuing to compete in evolutionary arms races.

While asexual reproduction is still utilized by multiple organisms, it appears that the advantages for sexual reproduction outweigh the advantages of asexual reproduction in a large array of circumstances. Asexual reproduction appears to be beneficial when the population size needs to be increased quickly. However, sexual reproduction appears to dominant in all other situations, including changing environment and evolutionary arms races, due to the genetic variation created. The interrelatedness between the hypotheses presented throughout the paper and the continuing amount of experiments being conducted on reproduction will allow researchers to further understand the importance that sexual reproduction has, not only on human society, but also on the entire ecosystem.

Works Cited

Carroll, L. 1871. Through the Looking-Glass. Macmillan Publishers, London.

Chao, L. 1990. Fitness of RNA virus decreased by Muller’s ratchet. Nature 348: 454-455.

Colegrave, N. 2002. Sex releases the speed limit on evolution. Nature 420: 664-666.

Crow, J.F. 1992. An Advantage of Sexual Reproduction in a Rapidly Changing Environment. Journal of Heredity 83: 169-173.

Howard, R.S, and C.M. Lively. 1997. The Maintenance of Sex by Parasitism and Mutation Accumulation under Epistatic Fitness Functions. Evolution 52: 604-610.

Kondrashov, A.S. 1988. Deleterious mutations and the evolution of sexual reproduction. Nature 336: 435-440.

Muller, H.J. 1964. The Relation of Recombination to Mutational Advance. Mutation Research 1: 2-9.

Van Valen, L. 1974. Molecular Evolution as Predicted by Natural Selection. Journal of Molecular Evolution 3: 89-101.

Vergara, D., J. Jokela, C.M. Lively. 2014. Infection Dynamics in Coexisting Sexual and Asexual Populations: Support for the Red Queen Hypothesis. The American Naturalist 184: S22-S30.

Revision to Wikipedia page

https://en.wikipedia.org/wiki/Evolution_of_sexual_reproduction#Evading_harmful_mutation_build-up

Inserted information: While DNA is able to recombine to modify alleles, DNA is also susceptible to mutations within the sequence that can affect an organism in a negative manner. Asexual organisms do not have the ability to recombine their genetic information to form new and differing alleles. Once a mutation occurs in the DNA or other genetic carrying sequence, there is no way for the mutation to be removed from the population until another mutation occurs that ultimately deletes the primary mutation. This is rare among organisms. Hermann Joseph Muller introduced the idea that mutations build up in asexual reproducing organisms. Muller described this occurrence by comparing the mutations that accumulate as a ratchet. Each mutation that arises in asexually reproducing organisms turns the ratchet once. The ratchet is unable to be rotated backwards, only forwards. The next mutation that occurs turns the ratchet once more. Additional mutations in a population continually turn the ratchet and the mutations, mostly deleterious, continually accumulate without recombination.[57] These mutations are passed onto the next generation because the offspring are exact genetic clones of their parent. The genetic load of organisms and their populations will increase due to the addition of multiple deleterious mutations and decrease the overall reproductive success and fitness.

For sexually reproducing populations, mutations in the DNA are more likely to be removed due to recombination in the process of meiosis. The offspring are also not direct genetic clones of a single parent. The alleles from both parents contribute to the offspring. This creates the ability to mask a mutation in the form of heterozygotes. Selection can also work in removing mutations from a sexual population. The lessened amounts of harmful mutations within an organism can lead to increased reproductive success. Natural selection will select for the reduced number of deleterious mutations. Many believe that this ability to evade the accumulation of harmful and possibly lethal mutations produces a substantial advantage for sexually reproducing populations.

Homework for 10-1

https://en.wikipedia.org/wiki/Sexual_reproduction

Suggestions: There should be a small focus on the different types of sexual reproduction. Some examples could be autogamy and allogamy. There could also be a discussion about internal and external reproduction.

Another addition could talk about how population size determines if sexual reproduction actually is beneficial in the ways that are suggested within this article. Certain articles suggest that population size determines how quickly an adaptation within a trait will be fixed in a population.

Finally, a section at the beginning of the article should be included about the start of life and explaining the use of asexual reproduction. This would give a perspective as to what occurred before the evolution of sexual reproduction and help to support the fact that sexual reproduction appears to be more widely beneficial than asexual reproduction for many organisms

Sentence added

While these ideas about why sexual reproduction has been maintained are generally supported, the ultimate size of the population determines if sexual reproduction is entirely beneficial. Larger populations appear to respond more quickly to benefits obtained through sexual reproduction than smaller population sizes.

Colegrave, N. (2012, September 24). Sex Releases the Speed Limit on Evolution. Nature. Retrieved September 12, 2014, from http://www.nature.com.proxy.lib.ohiostate.edu/nature/journal/v420/n6916/full/nature01191.html

Topic Why has sexual reproduction be selected for as a means for reproduction within many plants and animals?

Annotated Bibliography

Colegrave, N. (2012, September 24). Sex Releases the Speed Limit on Evolution. Nature. Retrieved September 12, 2014, from http://www.nature.com.proxy.lib.ohiostate.edu/nature/journal/v420/n6916/full/nature01191.html This article states that sexual reproduction may assist in helping fixation of certain genes to occur more rapidly. Ultimately this could help a species or population remove neutral or dangerous mutations quicker and help the population to increase its average fitness. I could use this article to show that populations that need to adapt quickly to a certain environment would most likely utilize sexual reproduction. This quick adaption could largely benefit the population and help the survival rate.

The Journal of Evolutionary Philosophy (n.d). The Evolution of Sexual Reproduction. The Journal of Evolutionary Philosophy. Retrieved September 14, 2014, from http://www.evolutionary-philosophy.net/sex.html This journal entry focuses on different theories as to why sexual reproduction has been favored throughout time, as organisms have become more complex. The main theory from the entry discusses that an increase in genetic recombination of organisms utilizing sexual reproduction allows natural selection to have options as to what genes are acted upon. This article will be a helpful background of different theories that are presented by different scientists. There is also some explanation as to why evolution would favor the system that creates more genetic recombination.

Lynch, M., Burger, R., Butcher, D. & Gabriel, W. The Mutational Meltdown In Asexual Populations. Journal of Heredity. 84 (5), 339-344. Retrieved September 14, 2014, from http://jhered.oxfordjournals.org.proxy.lib.ohio-state.edu/content/84/5/339.long This article focuses more on why asexual reproduction is disadvantageous for organisms. The main reason they propose is that individuals that use asexual reproduction have difficulty removing harmful mutations from their population. The harmful mutations are continually passed down to each generation because there is no mechanism for genetic recombination. The mutation must randomly remove itself, which could take many generations or may never occur. This article is a great source to show why evolution of reproduction shifted away from asexually and moved into a new direction, sexual reproduction.

Smith, J. M. (1978). The Evolution of Sex. Cambridge, UK: Cambridge University Press. This book discusses why he believes that sexual reproduction began to become popular within organisms. Not only does he discuss the advantages of sexual reproduction, he also gives his evidence of the origin of this type of reproduction. This would be a good source to use in the start of my paper. This could help give insight into the actual start of sexual reproduction, not only the reasons as to why it is advantageous. Williams, G.C. (1975). Sex and Evolution. Princeton,NJ: Princeton University Press. In this book, the most interesting metaphor used is discussion of lottery tickets. He asks the question in one chapter if it would be better to have multiple tickets with the same number (asexual reproduction) or multiple tickets with different numbers (sexual reproduction). This shows the general reasoning and understanding as to why most scientists believe that sexual reproduction is advantageous despite some of the energy costs. Using this metaphor or another similar metaphor would give a more relatable example as to why organisms and natural selection would choose to follow the path of sexual reproduction.