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Transcript
Natural Selection Biological Evolution and Classification Part I: Everybody in the Pool! Each of us has a variety of physical traits which our DNA helps to determine. Our genes are discrete DNA sequences which affect those traits. Alternative versions of genes are called alleles. Some alleles affect the physical traits that are inherited by children from their parents. A gene pool is the total collection of alleles within a population. Within a gene pool, some alleles are more frequent than others. Every species has its own gene pool which contains all the possible alleles which can impact on the various traits possessed by the species. For example, the color of your eyes is determined by the alleles you inherited from your mother and father. Each individual has two alleles for eye color. If your mother has one allele for blue eyes and one for brown, and your father has one allele for green eyes and one for brown, you may have brown eyes but carry a second allele for either blue, green, or brown eye color. Whichever combination of alleles you have, each of your offspring will inherit one of your alleles for eye color. Procedure: 1. In Part I of your Student Journal, write down your eye color. 2. When your teacher announces “GO,” find five different people in your class to survey. Ask their name and eye color as quickly as you can, then record that information in the data table found in your Student Journal. 3. Once you have recorded all of your data, complete the questions found below the data table in your Student Journal. Complete Part I of your Student Journal. 1 Natural Selection Biological Evolution and Classification Part II: Changes Over Time Biological evolution is based on changes in the frequencies of alleles from generation to generation. In some organisms, a single generation spans many years. Evolutionary time scales are generally measured in hundreds or even thousands of generations. Remember, an organism's genes generally do not change during the lifetime of the organism. Natural selection cannot change an individual. It can only select which individuals within a population have more or less reproductive success. A Case Study There are two variations of the European peppered moth (Biston betularia) in England: typica (“typical”), which are light colored, and carbonaria (“melanic”), which are dark colored. In the early 1800s, almost 99 percent of peppered moths near English cities were of the typica variety. Observations at the time noted that the light-colored moths were almost invisible when resting on trees that were covered by lichen. By the 1850s, however, coal-fueled factories were polluting the air with large amounts of black soot. The soot killed the lichen and blackened the tree trunks in industrial areas. European Peppered Moth Biston betularia By 1900, naturalist J.W. Tutt estimated that in the industrial city of Manchester, the percentage of carbonaria moths had grown from less than 1 percent of the total peppered moth population to over 98 percent. He hypothesized that this was due to higher predation by birds on the typica moths. Tutt's hypothesis was based on the theory of natural selection. According to the theory of natural selection, the change in frequencies of the different moth varieties could be caused by differential reproductive success driven by predation. Was this a case of natural selection at work? Variety: carbonaria Photo credit: Olaf Leillinger Please continue to the next page. 2 Natural Selection Biological Evolution and Classification Part II: Changes Over Time, continued In the late 1950s, British ecologist Bernard Kettlewell conducted an experiment to test Tutt's hypothesis. He raised both moth varieties in a laboratory, then marked the moths with a drop of paint so he could recapture the same moths after release. He then released several hundred of the moths in a wooded area near an industrial city where the tree trunks were darkened by pollution. During the day, he observed birds preying on the moths, taking more of the typica than the carbonaria moths. That night, he recaptured 27.5 percent of the carbonaria moths that he had released and 13.5 percent of the typica moths. A second trial was conducted that was identical to the first, except that it was performed in an unpolluted woodland area where the tree trunks were light colored. The results of this second experiment were the opposite of those of the first trial. Kettlewell observed higher predation by birds on the carbonaria moths than on the typica moths, and he recaptured more of the typica moths than the carbonaria moths at the end of the day. Kettlewell concluded that the moths that were more visible to the birds were more likely to be eaten, making the less-visible moths more likely to pass on their genes to the next generation. After decades of differential predation resulting in differential reproductive success, the initally rare, dark- colored carbonaria moths eventually became more and more common in industrialized areas. Kettlewell concluded that Tutt's hypothesis was valid, and that the changes in the moth populations were caused by natural selection. Flawed Science Exposed, Better Experimental Design Implemented Although his conclusions seemed sound, Kettlewell's experimental design was flawed. For example, he released and recaptured the moths on exposed tree trunks rather than on their natural resting places on trunk/branch joints and on branches. Furthermore, Kettlewell did not measure the abundance of lichen coverage in the different areas. This flaw became apparent years later when, after the rate of pollution was reduced, the proportion of typica moths overcame that of carbonaria moths before the lichens had returned. Years later, other scientists conducted their own experiments to try to explain the changes in moth varieties with changes in pollution. One such scientist was Michael Majerus, who conducted research from the mid-1960s to the 1990s. Using improved experimental methods and technologies, he (and others) obtained results supporting Kettlewell's conclusions, even though Kettlewell's own experiments are now considered to be too deeply flawed to do so themselves. Complete Part II of your Student Journal. 3 Natural Selection Biological Evolution and Classification Part III: Differential Reproductive Success A + B + C = Differential Reproductive Success A. Genetic variation Allele frequency changes in a population due to variation, or differences in alleles that are passed on. Different alleles are sometimes better suited to different environments than others. Allele frequencies change from generation to generation, as some are passed on more easily than others. The advantage or disadvantage of a given allele often depends on environmental conditions.. B. Potential of a population to produce more offspring than can survive (overpopulation) Each individual within a population will produce as many offspring as they can. This increases the chances that at least some of the offspring will survive to adulthood and reproduce. Hence, the population as a whole will often produce more offspring than the environment can support. C. Finite supply of environmental resources The environment provides a limited supply of food, water, shelter, and space. Moreover, the amount of each resource provided by the environment fluctuates or changes over time. Those that are best suited to obtain resources in their environment (best adapted) are more likely to survive and reproduce. = Differential Reproductive Success Some individuals/variants produce more offspring that survive to adulthood and reproduce than others. Those that are the most fit, or able to obtain resources and reproduce, tend to produce more offspring and thus pass on their alleles to the next generation. Please continue to the next page. 4 Natural Selection Biological Evolution and Classification Part III: Differential Reproductive Success, continued You are going to analyze how the ABC's of differential reproductive success work to produce changes in a population of organisms over time. Scenario: There are two islands consisting of true breeding tortoises. True breeding organisms are homozygous for a specific trait. Island A consists of true breeding long neck tortoises and Island B contains true breeding short neck tortoises. You are going to relocate tortoises from Island B to Island A. They will mate and help us model trait frequency. Procedure: Reproduction Rules 1. Obtain two sets of True Breeding Tortoises. Females are represented by one color of paper and the males by another color of paper. 2. Place six long neck and four short neck females in a plastic cup. Then place six long neck and four short neck males in another cup. 3. Without looking, select a male and a female tortoise from each cup to mate. 4. In Part III of your Student Journal, record the neck type of each parent and the resulting offspring. 5. Return the female piece to the cup containing the female breeding tortoises, and the male piece to the cup containing the male breeding tortoises. Each mated pair will produce 10 offspring from each trial. Dominant Phenotype: Long Neck Parents Long Neck Short Neck Long Neck NN Nn Short Neck Nn nn 6. Gently mix the pieces in each cup before repeating the breeding process for a total of eight trials. Then, answer questions 1-5 to evaluate the effects of the ABC's of natural selection on this population of tortoises. Complete Part III of your Student Journal. 5 Natural Selection Biological Evolution and Classification Part IV: From One Species to Another As you have learned, natural selection is a process that drives certain alleles to higher or lower frequencies within a population of organisms. If a population is divided, and the subpopulations remain isolated for a long period of time, then natural selection may increase and decrease the frequencies of different alleles in each subpopulation, causing the subpopulations to diverge from one another. If the genetic differences between the subpopulations become large enough, then the subpopulations become genetically incompatible and can no longer produce viable offspring by interbreeding. Speciation is the formation of new species via the divergence of subpopulations. Natural selection can contribute to speciation by causing the gene pools of subpopulations to become increasingly different from one another. In the 1830's when Charles Darwin visited the Galápagos Islands, the vice-governor accurately informed him that, just by looking at the shell of a tortoise, one could identify which island it came from. Although, they are identified as a “vulnerable species,” seven different subspecies of tortoises (Chelonoidis nigra) live in the wild on the Galápagos Islands today. In order to be considered part of the same species, two organisms must be able to breed and produce fertile offspring. Subspecies are populations of organisms that can interbreed, but they are unlikely to do so in nature as they are separated by geography or other factors. Over many generations, natural selection can cause one subspecies or the other to lose some alleles from their gene pool while the frequencies of other alleles increase. Eventually, each subspecies develops different traits that are unique to its gene pool, just like the shapes of the Galápagos tortoises' shells! To understand how natural selection could lead to the evolution of the many diverse subspecies of tortoise in the Galápagos Islands, it is necessary to know about the geography of the Galápagos Islands. Work on the questions in Part IV of your Student Journal and continue to the next page. 6 Natural Selection Biological Evolution and Classification Part IV: From One Species to Another, continued The Galápagos Islands are located about 600 miles west of Ecuador and sit right along the equator. They are a geologically young island chain, with the oldest (and largest) islands having formed around 5 million years ago. Scientists hypothesize that the original tortoises were carried to the Galápagos Islands by a strong current from the South American mainland. Tortoises do not swim, but they can float and are capable of going a long time without eating or having fresh water. Since Darwin's visit nearly 200 years ago, scientists have used DNA evidence to trace the genetic history of the tortoises. They have concluded that the larger islands were colonized by the original drifters. Over thousands of years, new islands formed that were later colonized by plants and other organisms. The tortoises gradually dispersed to the newer islands using the local currents over many generations. As moving between the islands was a rare event, each island colony had an extremely limited gene pool containing only a subset of the alleles from the gene pools on the larger islands. The current phenotypic diversity observed among the seven tortoise subspecies is due to the different frequencies of alleles among the isolated subpopulations on the different islands. The closest relative to the Galápagos tortoise is the Argentine tortoise (Chelonoidis chilensis), which is a smaller species of tortoise that lives on the mainland of South America. Complete Part IV and the Reflections and Conclusions of your Student Journal. 7