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5/24/2011 7:13:00 AM IB Standard level Biology Dulwich College Shanghai Topic 5: Ecology and Ecosystems Evolution (pg. 184-195) Define evolution (pg. 184) Outline the evidence for evolution provided by the fossil record, selective breeding of domestic animals and homologous structures. (pg. 185-186) State that populations tend to produce more offspring than the environment can support. (pg. 187) Explain that the consequence of the potential overproduction of offspring is a struggle for survival. (pg. 187) State that the members of a species show variation. (pg. 187) Explain how sexual reproduction promotes variation in a species. (pg. 187) Explain how natural selection leads to evolution. (pg. 187) Explain two examples of evolution in response to environmental change; one must be antibiotic resistance in bacteria. (pg. 191 & 194) 5/24/2011 7:13:00 AM IB Standard level Biology Dulwich College Shanghai Topic 5: Ecology and Ecosystems Evolution 5.4.1 Define evolution Orange book pg. 184 Green book pg. 85 Read the definition in your textbooks Do a brief search on the definition of ‘Evolution’. In your OWN words, try to define evolution. 5/24/2011 7:13:00 AM IB Standard level Biology Dulwich College Shanghai Topic 5: Ecology and Ecosystems Evolution Evidence 5.4.2 Outline the evidence for evolution provided by the fossil record, selective breeding of domestic animals and homologous structures. Orange book pg. 185-186 Green book pg. 85-86 1. Read the following information and then define these key words/terms: Fossil Fossilisation Dating rocks and fossils Relative dating Absolute dating Homologous structures 2. Outline how fossils (4 marks), homologous structures (6 marks) and selective breeding (4 marks) provide evidence for evolution. Fossil Record *Much of the information I have included here is extra background material to give you a better idea of fossils. www.museum.vic.gov.au/dinosaurs/fossintro.stm Most plants and animals that die are not fossilised. Their remains decay and are broken up and recycled by natural processes. Occasionally circumstances are right for fossilisation to occur. What is a Fossil? Fossils are remains moulds, or traces of organisms that died a long time ago and were preserved in (usually) sedimentary rocks. About 250 000 different fossil species have been identified. How are fossils formed? For millions of years, life was only found in the oceans. The oldest fossils are therefore of marine creatures. When animals died, their remains accumulated on the sea floor where they were buried by mud, sand or silt. When land animals or plants died, the soft parts usually decomposed or were eaten by scavengers. However, if the hard parts (bones, shells, wood) were covered by a sudden flood, or sand, or even volcanic ash, they might be preserved. Teeth are the hardest parts of an animal and were most likely to be preserved. Bone, wood and shell, although hard, have minute air spaces. When buried, water containing dissolved minerals may seep into these spaces and deposit minerals. Often, over millions of years, all of the original on or shell dissolves away leaving a complete mineral replacement embedded in the surrounding rock. The bones, wood and shell are then said to be petrified, or turned to stone. Fossils are found not only in rock: Extinct insects have been found in fossil tree sap Animals became trapped in natural tar pits and have been beautifully preserved Mammoths and other animals that lived during ice ages have been incorporated in ice, or frozen ground, so that flesh, hair and even stomach contents have been perfectly preserved. Sometimes, the entire animal decayed away but left a ‘mould’ that was then filled by sediments or minerals, making a natural ‘cast’. Similarly, footprints made in soft ground created moulds that were later filled, making casts. In some locations, impressions of skin have been discovered. Fossil skeletons are sometimes found relatively intact, although they have usually been scattered and only parts of the skeleton are recovered. Dating Rocks and Fossils The Earth is about 4600 million years old, the oldest fossils are about 3500 million years old, and the oldest fossils with hard parts are about 570 million years old. How do we know this? There are two methods by which we can estimate the age of rocks and the fossils contained in them. Relative Dating Rock dating is based on the study of rock strata and the fossils contained in each stratum. By examining quarries, cliffs, road cuttings and drill cores, a geologist can plot the different layers of rock that make up Earth’s crust. By assuming that any layer of rock is younger than the layer beneath it (except when the Earth’s crust has folded) ancient geological happenings were described sequentially using the names of the Geological Time Scale. Although the order of geological events could often be interpreted, the age in years was unknown. Absolute Dating Absolute dating is based on the rate of decay of radioactive elements in rocks. The discovery of radioactivity gave geologists a new ‘tool’ with which to measure the age of the Earth. Atoms of radioactive elements have unstable nuclei that progressively decay to a more stable form. Uranium-238 goes through many changes before the stable form, lead-206 is reached. The number refers to the number of particles in the nucleus. Forms of the same element may have nuclei with different numbers of neutrons and these are called isotopes. Careful laboratory study has shown that each radioactive element decays according to a distinct and measurable timetable. The number of decaying parent atoms continually decreases while the number of daughter atoms continually increases. The proportion, or percentage, of atoms that decay during one unit of time is constant. It has become common practice to designate that time unit in terms of the half life of the parent – meaning the time required to reduce the parent atom by one half. Palaeontologists have organised the results of their efforts in dating fossils into a Geological Time Scale. Most of us have heard of the Jurassic Period, but there are many other eras and periods. Selective Breeding Artificial selection (i.e. selective breeding) demonstrates the possibility that a selection pressure can cause dramatic changes in the phenotypes of animals/plants. People have developed many new varieties of plants and animals by selective breeding. Selection of specimens to breed based on particular traits is, in effect, changing the environment for the population. Those individuals lacking the desirable characteristics are not allowed to breed. Therefore, the following generations more commonly have the desired traits. Species that mature and reproduce large numbers in a short amount of time have a potential for very fast evolutionary changes. Insects and microorganisms often evolve at such rapid rates that our actions to combat them quickly lose their effectiveness. We must constantly develop new pesticides, antibiotics, and other measures in an ever escalating biological arms race with these creatures. Unfortunately, there are a few kinds of insects and microbes that are now significantly or completely resistant to our counter measures, and some of these species are responsible for devastating crop losses and deadly diseases. If evolution has occurred, there should be many anatomical similarities among varieties and species that have diverged from a common ancestor. Those species with the most recent common ancestor should share the most traits. For instance, the many anatomical similarities of wolves, dogs, and other members of the genus Canis are due to the fact that they are descended from the same ancient canine species. Wolves and dogs also share similarities with foxes, indicating a slightly more distant ancestor with them. Homologous Structures Homologous structures, like bones in the limbs of vertebrates, demonstrate a commonality in underlying structure even as the actual appendages are used for quite different things. The structure is called the pentadactyl limb (five fingered limb), eg. Porpoise fins, bat wing, mole legs, horse legs, human arms. This implies a common ancestry as, if each creature was specially created, why would such structures, so divergent in use and outer appearance, have the same bones, especially since we see analogous structures (structures used for similar purposes and that may look superficially similar, yet structurally are very different, eg. Wings of butterflies, bats and birds) that they could look like instead. 5/24/2011 7:13:00 AM IB Standard level Biology Dulwich College Shanghai Topic 5: Ecology and Ecosystems Evolution Overproduction State that populations tend to produce more offspring than the environment can support. Orange book pg. 187 Green book pg. 86 Read the following text then answer the questions at the end: The process of evolution by means of natural selection depends on a number of factors, including: “Organisms produce more offspring than can be supported by the available supply of food, light, space etc”. Darwin appreciated that all species have the potential to increase their number exponentially. To illustrate this point he wrote: “Suppose there are eight pairs of birds, and that only four of them annually …… rear only four young, and that these go on rearing their young at the same rate, then at the end of seven years there will be 2048 birds instead of the original 16”. Darwin realised that, in nature, populations rarely, it ever, increased in size at such a rate. He rightly concluded that the death rate of even the most slow-breeding species must be extremely high. Why and how do organisms reproduce so rapidly with little prospect of all but a tiny proportion of offspring surviving? The reason why reproduction rates are high is because a species cannot control the climate, rate of predation, availability of food etc. Therefore to ensure a sufficiently large survives to breed and produce the next generation, each species must produce vast numbers of offspring. This is to compensate for considerable death rates from predation, lack of food (including light in plants) and water, extremes of temperature, natural disasters such as earthquakes and fire and disease. How organism over-produce depends on the species in question and its means of reproduction, some examples include: A bacterium can divide by binary fission about every 20 minutes when conditions are favourable. A single bacterium could therefore give rise to 4 x 1021 in just 24 hours. Some fungi can produce over 500 000 spores each minute at the peak of production. Each spore has the potential to form a new fungal mycelium. Higher plants can spread rapidly by vegetative propagation, e.g. the produce of bulbs, rhizomes and runners. Flowering plants produce vast amounts of pollen from their anthers. These can fetilise the many ovules in plants of the same species, leading to the production, in some cases, of millions of seeds from a single parent. Animals produce vast numbers of sperm, and sometimes large number of eggs also. A female oyster, for example, can produce 100 million eggs in a year and the male oyster produces many more times this number of sperm. Many organisms, e.g. birds such as finches and mammals like the rabbit, produce several clutches/ litters every year, each of which comprises several offspring. The importance of over-production to natural selection lies in the fact that, where there are too many offspring for the available resources, there is competition amongst individuals (intraspecific competition) for the limited resources available. The greater the numbers, the greater this competition and the more individuals will die in the struggle to survive. These death are, however, not random. Those individuals best suited to prevailing conditions (e.g. better able to hide from or escape predators, better able to obtain light or catch prey, better able to resist disease or find a mate) will be more likely to survive than those less well adapted. These individuals will be more likely to breed and so pass on these favourable characteristics, via there alleles, to the next generation, which will therefore be slightly different from the previous one – i.e. the species will have evolved to be better adapted to the prevailing conditions. The selection process, however, depends on individuals of a species being genetically different from one another. Questions: 1. According to Darwin’s theory of evolution, what causes the struggle for survival in populations? (1 Mark) A. Overproduction of offspring B. Favourable heritable variations C. Natural selection D. Competition between the fittest individuals in the population 2. Why and how do organisms reproduce so rapidly with little prospect of all but a tiny proportion of offspring surviving? (3 marks) 3. Where there are too many offspring for the available resources, there is competition amongst individuals- Describe what is meant by competition giving at least 2 examples. (3 marks). 5/24/2011 7:13:00 AM IB Standard level Biology Dulwich College Shanghai Topic 5: Ecology and Ecosystems Evolution Struggle for Survival Explain that the consequence of the potential overproduction of offspring is a struggle for survival. Orange book pg. 187 Green book pg. 86 Darwin's Theory of Evolution by Natural Selection Darwin’s theory of natural solution is so simple that when Darwin’s close friend, T. H. Huxley, read of it, he said ‘how stupid of me not to have thought of it first’. The theory can be summarized by means of four hypotheses which result in two conclusions: Hypothesis 1: Individuals within a species differ from each other - there is variation. Hypothesis 2: inherited. Offspring resemble their parents- characteristics are Hypothesis 3: Far more offspring are generally produced than survive to maturity - they suffer from predation, disease and competition. (Overproduction) Hypothesis 4: There is a struggle for survival; some individuals being better adapted to their environment and therefore more successful than others. Conclusion 1: The better adapted individuals that survive and reproduce pass on their beneficial characteristics to their offspring. Conclusion 2: In time, the individuals in a species may give rise to a new collection of individuals that are sufficiently distinct to be classified as a separate species. Darwin concluded that individuals that were better adapted to their environment compete better than the others, survive longer and reproduce more, so passing on more of their successful characteristics to the next generation. Darwin used the memorable phrases survival of the fittest, struggle for existence and natural selection. Darwin explained the giraffe's long neck as follows. In a population of horse-like animals there would be random genetic variation in neck length. In an environment where there were trees and bushes, the longer-necked animals were better adapted and so competed well compared to their shorter-necked relatives. These animals lived longer, through more breeding seasons, and so had more offspring. So in the next generation there were more long-neck genes than short-neck genes in the population. If this continued over very many generations, then in time the average neck length would increase. Today it is thought more likely that the selection was for long legs to run away from predators faster, and if you have long legs you need a long neck to be able to drink. But the process of selection is just the same. Darwin wasn't the first to suggest evolution of species, but he was the first to suggest a plausible mechanism for the evolution - natural selection, and to provide a wealth of evidence for it. Darwin used the analogy of selective breeding (or artificial selection) to explain natural selection. In selective breeding, desirable characteristics are chosen by humans, and only those individuals with the best characteristics are used for breeding. In this way species can be changed over a long period of time. All domesticated species of animal and plant have been selectively bred like this, often for thousands of years, so that most of the animals and plants we are most familiar with are not really natural and are nothing like their wild relatives (if any exist). The analogy between artificial and natural selection is a very good one, but there is one important different - Humans have a goal in mind, nature does not. 5/24/2011 7:13:00 AM IB Standard level Biology Dulwich College Shanghai Topic 5: Ecology and Ecosystems Evolution Variation State that the members of a species show variation. Orange book pg. 187 Green book pg. 87 Read the text below then answer the questions that follow: If there was no variation between the individuals within a species it is easy to see that selection would not take place. Identical organisms would all have the same characteristics that could be selected for or against and hence distinguishing one organism from another as having an evolutionary advantage would not be possible. Since all individuals within a population show variation and a ‘struggle for existence’, has been clearly established, it follows that some individuals possessing particular variations will be more suited to survive and reproduce. The key factor in determining survival is adaptation to the environment. Any variation, however slight, be it physical, physiological or behavioural, which gives one organism an advantage over another organism will act as a selective advantage in the ‘struggle for existence’. Favourable variations will be inherited by the next generation. Unfavourable variations are ‘selected out’ or ‘selected against’, their presence conferring a selective disadvantage on that organism. If an organism can survive in the conditions in which it lives, you may wonder why it doesn’t produce offspring that are identical to itself. These will, after all, be equally capable of survival in these conditions, whereas variation may produce individuals that are less suited. However, conditions change over time and having a wide range of different individuals in the population means that some will have the combination of genes needed to survive in almost any set of new circumstances. Populations showing little individual variation are vulnerable to new diseases and climate change. It is also important that a species adapts to changes resulting from the evolution of other species. If, for example, rabbits in a particular region adapt to run faster, foxes and other predators will be less able to catch them and therefore have less food, unless they in turn develop greater speed. A species cannot predict future changes; it does not know whether the climate will become wetter/ drier, warmer/ colder or how its prey or predator will evolve or what new disease agent may arise. However, the larger the population is, and the more genetically varied the organism within it, the greater the chance that one or more individuals will have the genetic characteristics that give it an advantage in the struggle for survival. These individuals will therefore be more likely to breed and pass their more suitable characteristics on to future generations. Variation therefore provided the potential for a species to evolve and so adapt to new circumstances. Questions: 1. Define ‘Variation’ and ‘selection advantage’. 2. Describe how unfavourable variations/characteristics are selected against within a population. 3. If an organism can survive in the conditions in which it lives, why doesn’t nature allow it to produce offspring that are identical to itself? 5/24/2011 7:13:00 AM IB Standard level Biology Dulwich College Shanghai Topic 5: Ecology and Ecosystems Evolution Sexual Reproduction and Variation Explain how sexual reproduction promotes variation in a species. Orange book pg. 187 Green book pg. 87 Read the information below and define the following key terms: Independent assortment Crossing over in meiosis Random fertilisation Genetic Variation in Sexual Reproduction The whole point of meiosis and sex is to introduce genetic variation, which allows species to adapt to their environment and so to evolve. There are three sources of genetic variation in sexual reproduction: Independent assortment in meiosis Crossing over in meiosis Random fertilisation Independent Assortment This happens in meiosis, when the chromosomes line up on the equator. They line up as two homologous chromosomes, which originally came from two different parents (they’re often called maternal and paternal chromosomes). Since they can line up in any orientation on the equator, the maternal and paternal versions of the different chromosomes can be mixed up in the final gametes. In this simple example with 2 homologous chromosomes (n=2) there are 4 possible different gametes (22). In humans with n=23 there are over 8 million possible different gametes (223). Crossing Over This happens during meiosis when the homologous pairs line up together. While the two homologous chromosomes are joined, bits of one chromosome are swapped (crossed over) with the corresponding bits of the other chromosome. The points at which the chromosomes actually cross over are called chiasmata (singular chiasma). There is always at least one chiasma in a bivalent, but there are usually many. Crossing over means that maternal and paternal alleles can be mixed, even though they are on the same chromosome. Random Fertilisation This takes place when two gametes fuse to form a zygote. Each gamete has a unique combination of genes, and any of the numerous male gametes can fertilise any of the numerous female gametes. So every zygote is unique. 5/24/2011 7:13:00 AM IB Standard level Biology Dulwich College Shanghai Topic 5: Ecology and Ecosystems Evolution Natural Selection Explain how natural selection leads to evolution. Orange book pg.187 Green book pg. 87 Read the text and then answer the questions that follow: *This is a recap on notes you have already read, the idea of natural selection and how it brings about evolution should becoming clearer to you now. Darwin's Theory of Evolution by Natural Selection Darwin’s theory of natural solution is so simple that when Darwin’s close friend, T. H. Huxley, read of it, he said ‘how stupid of me not to have thought of it first’. The theory can be summarized by means of four hypotheses which result in two conclusions: Hypothesis 1: is variation. Individuals within a species differ from each other - there Hypothesis 2: inherited. Offspring resemble their parents- characteristics are Hypothesis 3: Far more offspring are generally produced than survive to maturity - they suffer from predation, disease and competition. Hypothesis 4: There is a struggle for existence; some individuals being better adapted to their environment and therefore more successful than others. Conclusion 1: The better adapted individuals that survive and reproduce pass on their beneficial characteristics to their offspring. Conclusion 2: In time, the individuals in a species may give rise to a new collection of individuals that are sufficiently distinct to be classified as a separate species. Darwin concluded that individuals that were better adapted to their environment compete better than the others, survive longer and reproduce more, so passing on more of their successful characteristics to the next generation. Darwin used the memorable phrases survival of the fittest, struggle for existence and natural selection. Darwin explained the giraffe's long neck as follows. In a population of horse-like animals there would be random genetic variation in neck length. In an environment where there were trees and bushes, the longer-necked animals were better adapted and so competed well compared to their shorter-necked relatives. These animals lived longer, through more breeding seasons, and so had more offspring. So in the next generation there were more long-neck genes than short-neck genes in the population. If this continued over very many generations, then in time the average neck length would increase. Today it is thought more likely that the selection was for long legs to run away from predators faster, and if you have long legs you need a long neck to be able to drink. But the process of selection is just the same. Darwin wasn't the first to suggest evolution of species, but he was the first to suggest a plausible mechanism for the evolution - natural selection, and to provide a wealth of evidence for it. Darwin used the analogy of selective breeding (or artificial selection) to explain natural selection. In selective breeding, desirable characteristics are chosen by humans, and only those individuals with the best characteristics are used for breeding. In this way species can be changed over a long period of time. All domesticated species of animal and plant have been selectively bred like this, often for thousands of years, so that most of the animals and plants we are most familiar with are not really natural and are nothing like their wild relatives (if any exist). The analogy between artificial and natural selection is a very good one, but there is one important different - Humans have a goal in mind, nature does not. Questions: 1. Which process has the greatest effect in determining which members of a population are most likely to survive until reproductive age? A. Evolution B. Natural selection C. Meiosis D. Hybridization 2. Which factors could be important for a species to evolve by natural selection? I. Environmental change II. Inbreeding III. Variation A. I only B. I and II only C. I and III only D. I, II and III 3. What is natural selection? A. The mechanism that increases the chance of certain individuals reproducing. B. The mechanism that leads to increasing variation within a population. C. The cumulative change in the heritable characteristics of a population. D. The mechanism that explains why populations produce more offspring than the environment can support. 5/24/2011 7:13:00 AM IB Standard level Biology Dulwich College Shanghai Topic 5: Ecology and Ecosystems Evolution Examples of Evolution Explain two examples of evolution in response to environmental change; one must be antibiotic resistance in bacteria. Orange book pg. 191 & 194 Green book pg. 88 Read the example below and then summarise how antibiotic resistance in bacteria is an example of evolution in response to environmental change (Use your class notes on DDT resisitance in mosquitos/peppered moth melanism as a template for structuring your answer). Development of Antibiotic Resistance How do bacteria become resistant to antibiotics? Some species may be resistant because they don't posses the antibiotic target. E.g. penicillin targets the cell wall and weakens it by inhibiting the enzymes which make peptidoglycan for the cell wall. The bacteria which causes Chlamydia has a cell wall that does not contain with no peptidoglycan and hence penicillin will not act on it. In general, resistance first develops due to a mutation. Bacteria reproduce asexually, so all the offspring should be the same, but sometimes, at random, mutations occur when DNA is replicated. These mutations may have any effect (and most will be fatal), but just occasionally a mutation occurs that makes that bacterium resistant to an antibiotic. For example a mutation could slightly alter a protein so that an antibiotic can no longer bind, or a mutation could slightly alter an enzyme, changing its substrate specificity so that its active site will now bind penicillin. Such mutations are very rare, but bacteria reproduce so rapidly, and there are so many bacterial cells, that new resistance mutations do crop up at a significant rate (a few times per year somewhere on the planet). Remember that development of antibiotic resistance is a random event, and is not caused by the presence of the antibiotic. It is certainly not an adaptation that bacteria acquire. As an example, imagine a community of different bacterial species living in your gut, and one particular cell has just mutated to become resistant to penicillin. What happens next? It will reproduce by binary fission and pass on its resistance gene to all its offspring, forming a new strain of bacteria in your gut. If there is no antibiotic present in your gut (most likely) this mutated strain may well die out due to competition with all the other bacteria, and the mutation will be lost again. However, if you are taking penicillin, then penicillin will be present in the bacteria's environment, and these mutated cells are now at a selective advantage: the antibiotic kills all the normal bacterial cells, leaving only the mutant cells alive. These cells can then reproduce rapidly without competition and will colonise the whole environment. This a good example of natural selection at work. Spread of Antibiotic Resistance How do these resistant bacteria spread to other people? In the example above the bacteria will contaminate faeces and may then infect other individuals through poor hygiene. In fact the resistant bacteria can spread by any of the normal methods of spreading an infection: through water, food, sneezing, infected instruments, etc. Some bacteria can form spores to aid their dispersal, and so a mutated strain can survive long journeys and long periods of time. In most new environments the mutated strain will die out through competition, but whenever it encounters penicillin it will thrive, out-competing all other bacteria. This is how resistance of one bacterial strain to one antibiotic can spread, but unfortunately resistance can also spread between species. Bacteria have a trick that no other organisms can do: they can transfer genes between each other; even between different species. In this way a resistance gene can spread from the bacterium in which it arose to other, perhaps more dangerous, species. There are three methods of gene transfer in bacteria: Conjugation This is the transfer of DNA between bacterial cells via a cytoplasmic bridge. From time to time two bacterial cells can join together (conjugate), and DNA passes from one (the donor) to the other (the recipient). The transferred DNA can be one or more plasmids, or can be all or part of the whole bacterial chromosome (in which case the donor cell dies). Conjugation is sometimes referred to as bacterial sex or mating, but it is quite distinct from sexual reproduction, because the gene exchange is not equal, it can take place between different species, and bacteria do not use conjugation for reproduction. It is better thought of as an alternative to sex, where these asexual organisms gain some of the advantages of genetic exchange. Evolution essay 5/24/2011 7:13:00 AM Essay title: Explain evolution of a species by natural selection using a named example (18 marks) Your essay should be hand written.