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Reproduction is the production of new individuals by an existing member or members of the same species. It is a fundamental characteristic of living things; the ability to self replicate in this way sets the living world apart from the non-living. Sexual reproduction is the exclusive form of reproduction in all higher invertebrates and in all vertebrates, whereas in few parthenogenesis is also possible. Also it occurs in many multicellular and in all unicellular organisms such as plants and animals. In sexual reproduction one sex cell (egg) fuses with another sex cell (sperm) producing a new cell called zygote. Fusion of gametes or sex cells is called fertilization. In the process of gamete formation nuclear division known as meiosis halves the normal chromosome number. That is gametes are haploid and fertilization restores the diploid number of chromosomes. There are many advantages and disadvantages in sexual reproduction; these will be discussed further in this essay.
Figure 1: General animal life cycle.
Advantages of sexual reproduction
One of the most important advantages of sexual reproduction is that it results in genetic variation among offsprings. This is a result of meiosis, which is the halving of the number of chromosomes and fertilization, whereby the male and female sex cells fuse and the restoration of the original number of chromosomes. Among them meiosis is the major source of genetic variation. This genotypic variation is caused due to the two events; independent assortment and crossing over that take place during meiosis. Independent assortment occurs because the way the bivalents line up at the equator of the spindle during meiosis Ðâ€ is entirely random. Therefore, the number of possible combinations of chromosomes that can be formed by random orientation in meiosis is 2n (n= number of pairs of chromosomes). There is also crossing over of segments of individual maternal and paternal homologous chromosomes (Hered, 1992).These events results in new combinations of genes on the chromosomes of the haploid cells produced. According to Hered (1992) these variations found in offspring of sexual reproduction allow some individuals to be better suited for survival and provide a mechanism for selective adaptation to occur.
Genetic recombination is another advantage of sexual reproduction. According to Antonovics & Kelly (1988) genetic recombination helps to separate beneficial mutations from harmful mutations. An organism can be subjected to many changes due to mutation. Most of the non-neutral mutations are known to be deleterious and this means that they will cause a decrease in organisms overall fitness. Mutations with deleterious effects can be removed from the population by natural selection (Antonovics & Kelly, 1988).It is believed that natural selection acts upon the entire genetic makeup of an organism without sexual reproduction, but with sexual reproduction natural selection acts upon individual genetic traits.
Sexual reproduction also helps in the spread of advantageous traits. In sexual reproduction novel genotype are produced. As mentioned earlier since this type of reproduction is due to the fusion of two gametes with different combination of genes, populations are capable of combining advantageous genes than in asexual (Crow, 2005). Sexual reproduction increases resistance to parasites, this is also known as the Red Queen hypothesis. The Red Queen refers to the frequency-dependent relationship between host and parasites. It shows those sexual hosts are continually adapting in order to stay ahead of their parasites. Thereby, this clearly illustrates that sexual reproduction helps to increase parasitic resistance (Kang, 2008,).
Disadvntages of sexual reproduction
However, sexual reproduction is not the perfect reproductive method if it examined in detail there are many disadvantages which are considerable. As mentioned by Graham (1982) one f the draw backs of sexual reproduction is that the cost of reproducing offspring by means of sexual reproduction is almost twice the cost of reproducing asexually. That is while sexual lineage is completing one division cycle, asexual lineage could finish several division cycles. This is because meiosis takes considerably longer time than two mitotic divisions. The processes of syngamyi and karyogamy (combining of gametic nuclei) also require a longer period. As explained by Crow (2005) Chromosomes recombination and independent assortment are the major features of meiosis and as a result of these features the allelic combinations can be disrupted within and between chromosomes. In other words, in sexual reproduction they are incapable of producing clones as offspring. Therefore, because of this process it is likely that high fitness genotypes will be destroyed and the mean fitness of the offspring will be reduced.
Another disadvantage is the additional expenditure of time and energy needed to attract or find mates. Also during that time spent an individual may encounter predators (Campbell & Reece, 2008). Furthermore, as pointed out Kondrashov (2001) ââ‚¬Å“cost of meiosis or 50% cost, shows that for example if there is, a population of 100 asexual and 100 sexual individuals in the same environment, while only 50% of the sexual population can produce offsprings, all the asexual population will be able to produce offsprings. This is known as 2:1 demography disadvantage.
Unlike in asexual reproduction there is another huge risk of exposure to sexually transmitted diseases during sexual reproduction in animals as well. It is clearly said that the time during mating is the ideal time for the transmittance of parasites and pathogens form one individual to another.The most common sexually transmitted disease among animals today is brucellosis (undulant fever).This is common among animals such as dogs, goat, rats and domestic livestocks. (Oriel & Hayward, 1974).
In conclusion, sexual reproduction brings about many advantages such as genetic variation, recombination function to eliminate mutations, also increase resistance to parasites and producing organisms different to each other. Though sexual reproduction results in disadvantages such as energy wasted during searching for a mate, time consumed during long cell cycles. Overall when it is compared to asexual reproduction, sexual reproduction results in far more advantages in the long run.