Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Costs and benefits of sexual and asexual reproduction: CfE Advanced Higher Biology Unit 2: Organisms and Evolution 2.3 Variation and sexual reproduction a) Costs and benefits of sexual and asexual reproduction b) Meiosis forms viable gametes c) Sex determination Key areas • Describe the costs and benefits of sexual and asexual reproduction. • Compare sexual and asexual reproduction. • Describe advantages and disadvantages to both processes. • Link the processes to the red queen hypothesis. • Define parthenogenesis and give examples of organisms that use this process. • Identify the use of horizontal gene transfer in asexual reproduction. Starter Which organisms use sexual or asexual reproduction? Sexual and asexual reproduction • How do we define each? • Sexual- offspring arise from the combination of male and female gametes after the process of fertilisation. • Asexual- offspring arise from a single organism and share the DNA of that parent only. Offspring can be classed as clones. • What are the advantages and disadvantages of both? Sexual reproduction has 2 major disadvantages: • 1. Only half of a genome is passed on from each parent. This means that a “successful” parent can only pass on half of their “successful” genes, as their genetic information is mixed with the genetic information of another individual. • 2. Only half of the population produce offspring (males don’t reproduce). This is known as the paradox of the existence of males. The paradox of the existence of males • In any sexually reproducing species, half of any offspring produced will be male and half female. • However, it is only the females who are able to reproduce the next generation. • In an asexually reproducing species, all offspring are able to reproduce and would be expected to increase in number at twice the rate of a sexually reproducing species. The paradox of the existence of males • The fact that the males in a sexually reproducing organism are not able to reproduce would appear to put sexually reproducing species at a disadvantage. • In terms of the number of offspring they are able to produce and, they would be outcompeted by asexual species. This is known as the paradox of the existence of males. Sexual and asexual reproduction Asexual reproduction Sexual reproduction Asexual populations rapidly outgrow sexual ones as males in BLUE do not reproduce offspring. Genome disruption • Sexual reproduction requires that the genomes of both parents (male and female) are disrupted in order to half the number of chromosomes present in each gamete. In all sexually reproducing eukaryotes, this involves the production of gametes through the process of meiosis. Human male genome • By its very nature, only half of each of these genomes will be passed on and which alleles are present in each gamete is random as a result of crossing over and independent assortment. The benefits of sexual reproduction • As sexual reproduction is so widespread across the eukaryotic kingdoms, the benefits must outweigh the disadvantages. The benefits: • Greater genetic variation which allows species to maintain a large gene pool. • Genetic variation allows organisms to cope with selection pressures and evolve to changing environmental situations. • It provides the raw material required to keep running in the Red Queen’s ‘arms race’ between parasites and their hosts. It is important to remember that evolution itself does not have a goal or a direction. Sexual Reproduction Key Concepts • Disadvantages of sexual reproduction: males unable to produce offspring; only half of each parent’s genome passed onto offspring disrupting successful parental genomes. • Benefits outweigh disadvantages due to increase in genetic variation in the population. • This genetic variation provides the raw material required to keep running in the Red Queen’s arms race between parasites and their hosts. Asexual reproduction • Asexual reproduction avoids both of the disadvantages of sexual reproduction, but does not give the same amount of variation. • It can be an effective reproductive strategy, particularly in very narrow, stable niches or when recolonising disturbed habitats. • This type of reproduction allows offspring to be produced quickly (allowing quick colonisation) in the absence of a mate. Sea anemone with asexually reproduced offspring Asexual reproduction- Prokaryotes/ Unicellular organisms • In prokaryotes, cell division produces new individuals. To increase the flow of genetic information horizontal gene transfer via plasmids is often used. Mechanism of asexual reproduction – Fission In the process of fission, the parent cell is replaced by two daughter cells as it literally splits into two cells of approximately equal size. Example of Archaea domain - Halobacter Example of the Protist Kingdom - Acathocystis turfacea Example of bacteria domain – Staphylococcus aureus Asexual reproduction – Eukaryotes Mechanisms of asexual reproduction in Eukaryotes Vegetative cloning – in plants. eg. production of rhizomes (ginger) or runners (strawberry) or bulbs (daffodils) or tubers (dahlia) Parthenogenesis – a type of asexual reproduction in which an unfertilised egg develops into a new individual eg . fire ants. • In eukaryotic plants there are many successful forms of vegetative cloning. – Plantlets on runners e.g. strawberry – Leaf edge plantlets e.g. Mexican hat plant – Offsets e.g water lettuce (pistia) – Tubers e.g sweet potato Parthenogeneis • In eukaryotic animals another successful asexual reproductive strategy is Parthenogenesis. • Parthenogenesis involves producing an individual from an unfertilised egg, the word is derived from the Greek for ‘virgin birth’. • Individuals produced can therefore be haploid or diploid. Parthenogenesis • Adults produced are usually haploid and their cells don’t undergo meiosis in forming new eggs. • Parthenogenesis is more common in cooler climates with low parasite diversity. Examples of parthenogenesis In social insects e.g. bees, wasps or ants, males are produced from unfertilised eggs and so are haploid individuals. Sterile workers and reproductive workers (Queens) are produced from fertilized eggs. • Some species of Whiptail Lizards go through mating behaviours and hormone cycles similar to that of organisms that undergo sexual reproduction. The desert grassland whiptail lizard is an all female species. • They reproduce by parthenogenesis. The chromosomes within their eggs double after meiosis, thus allowing them to develop into lizards without fertilisation occurring. Examples of parthenogenesis Daphnia have the ability to switch between sexual and asexual reproduction. The method is often related to the season; asexual occurring when conditions are favourable and sexual occurring during times of environmental stress. Horizontal gene transfer For the many organisms for whom asexual reproduction is the principal method of reproduction, many have mechanisms for horizontal gene transfer between individuals, such as the plasmids of bacteria and yeast. Main mechanisms of horizontal gene transfer: •Transformation •Transduction •Conjugation Asexual Reproduction Key Concepts • Asexual reproduction can be a successful reproductive strategy, particularly in very narrow, stable niches or when recolonising disturbed habitats. • In eukaryotes, examples of asexual reproduction include vegetative cloning in plants and parthenogenic animals that lack fertilisation. • Parthenogenesis is more common in cooler climates that are disadvantageous to parasites or regions of low parasite density/diversity. • Organisms that reproduce principally by asexual reproduction often have mechanisms for horizontal gene transfer between individuals, such as the plasmids of bacteria and yeast. Homework Task • Using the scientific paper take notes about the evolution of sexual and asexual reproduction (reference your notes). • The whole paper will need to be read to help you answer the questions. • Further reading: • http://www.trueorigin.org/sex01.asp • http://www.nature.com/scitable/topicpage/sexual -reproduction-and-the-evolution-of-sex-824 Past paper questions