DISCUSS THE ADVANTAGES AND DISADVANTAGES OF SEXUAL AND ASEXUAL REPRODUCTION
DISCUSS THE ADVANTAGES AND DISADVANTAGES OF SEXUAL AND ASEXUAL REPRODUCTION
In order to avoid extinction species must reproduce and pass their genes on to the next generation, thus ensuring the survival of the population. For reproduction to occur cell division is necessary and this can be divided into two categories according to the behaviour of the chromosomes, they are known as mitosis and meiosis. Mitosis is when the daughter cells finish up containing exactly the same number of chromosomes as the parent cell, typically two of each type, known as the diploid state. Mitosis takes place when an organism grows or reproduces asexually. Meiosis is when each daughter cell ends up with exactly half the number of chromosomes as the parent cell, one of each type, which is known as the haploid state. Meiosis is associated with sexual reproduction and generally takes place in the formation of gametes or in some cases' spores.
Asexual reproduction is the process of one individual organism producing one or more new individuals. It does not involve the combination of genetic material from two different individuals and therefore the new individuals are clones of the parent organism. Mutation is the only variation that occurs in asexual reproduction, unless mutation occurs all the members of clones share the same genetic constitution. Sexual reproduction involves the fusion of gametes, this is known as fertilisation. The genetic material from two different individuals is mixed and combined to produce an individual that is genetically different from either parent. All individuals, unless they are identical twins are genetically unique, this process produces variation within the population. Many organisms that carry out asexual reproduction also carry out sexual reproduction at some stage in their lifecycle, hardly any animal species use it as their only means of reproduction.
Asexual reproduction in animals has three main methods of reproducing, which are budding, parthenogenesis and regeneration. Budding simply consists of an organism developing an outgrowth which on detachment forms a new self supporting individual. Parthenogenesis or "virgin birth" is when unfertilised eggs develop into new individuals and regeneration happens when an organism's body is broken into fragments, then each fragment can then develop into a new individual. Using these methods avoids the need to find a mate and also rapidly increases the number of organisms in a population, all of which are advantages of asexual reproduction. However, these ...
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Asexual reproduction in animals has three main methods of reproducing, which are budding, parthenogenesis and regeneration. Budding simply consists of an organism developing an outgrowth which on detachment forms a new self supporting individual. Parthenogenesis or "virgin birth" is when unfertilised eggs develop into new individuals and regeneration happens when an organism's body is broken into fragments, then each fragment can then develop into a new individual. Using these methods avoids the need to find a mate and also rapidly increases the number of organisms in a population, all of which are advantages of asexual reproduction. However, these advantages only work well if the environment is favourable and does not change. Disadvantages associated with these methods are that because the organisms that are produced are all identical there is less genetic variation and the population can not adapt to change, therefore making evolution slower. Also, because the organisms are clones it is likely that all the organisms would die if a serious disease came along. Inbreeding also causes genetic defects and diseases.
Many flowering plants can reproduce asexual as well as sexually. Asexual reproduction allows plants to increase their numbers or to survive between growing seasons. It gives them extra flexibility and allows them to reproduce and live even outside the flowering season. Vegetative propagation produces new plants without forming flowers and seeds. A single parent has offspring that are genetically identical, both to it and to each other. Other plants can regenerate or repair themselves, stems or leaves that are broken off can root and grow into new plants. By being able to reproduce asexually as well as sexually they have adapted themselves and taken advantage of nature to make sure they have a better chance of survival.
In sexual reproduction there are two forms of fertilisation, cross fertilisation where the gametes come from two separate individuals and self fertilisation where both gametes come from the same individual. As plants can not move they use both forms of fertilisation and they have evolved many wonderful strategies to make sure that their genes pass to the next generation. Flowering plants have male and female sex organs which produce male and female spores. The male parts produce pollen and the female parts produce megaspores, pollination occurs when the pollen reaches the female parts of the plant resulting in fertilisation. Pollination occurs by wind or vectors. Self fertilisation is used as a back up, only adopted if cross fertilisation fails to occur for some reason.
In most organisms, apart from bacteria, sexual reproduction involves the union of gametes. In some environments the mating of animals is dangerous due to visibility from predators or aggression from competitors or even the mate. Mating is a slow process and there is no guarantee of conception, only half the copies of the genes are reproduced in each offspring compared to whole, exact copies of asexual reproduction. Why then with so many disadvantages do animals reproduce sexually? The answer is quite simple, variation, this arises from individual genes mutating, often caused by faults in the copying of DNA. Once there is some variation to work on the differences are maximised by the process of sexual reproduction.
Variation is necessary for survival, it greatly increases the chance that at least some individuals will adapt to changing conditions. Producing genetically variable offspring allows evolutionary adaptation to inter species competitions, individuals stand a better chance of staying ahead in the "genetic arm's race" against evolutionary development in predators, prey and parasites. It also gives a genetic advantage of aiding DNA repair and preventing maladaptive mutations affecting all offspring and allows the easier expression of favourable recessive genes. Producing genetically different offspring also reduces sibling competition for the same environment, this may occur in asexual reproduction. conditions. Variation also increases the chance that some individuals could be resistant to disease, therefore surviving to breed, ensuring that the population and perhaps the whole species continue to exist. Together with natural selection, variation in the long term is the main driving force behind the process of evolution.
There are many advantages and disadvantages to both asexual and sexual reproduction, the main advantages being the rapid increase of population and variation respectively. In my own opinion it is the organisms that can produce both asexually and sexually that have the advantage over other organisms that use only one form of reproduction. They can use sexual reproduction to gain variation that is needed for survival and they can use asexual reproduction to increase the population rapidly, and as a back up if sexual reproduction fails. To illustrate the pros and cons of both types of reproduction I would use the aphid as an example. They use asexual reproduction to give birth to wingless females which are born pregnant, they in turn give birth to winged females who can fly away and colonise new plants. As the days grow shorter towards the end of the summer aphids of both sexes are born who then reproduce sexually, laying their eggs which survive the winter to start the whole process again. We only have to go into our gardens in the summer to see how successful this process is as every garden will have at least some aphids present.
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