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- Students will demonstrate the randomness associated with the passing of alleles from parents to offspring.
- Students will know the concept of incomplete dominance.
- Students will review the concepts of dominant and recessive alleles and genotypes and phenotypes.
- Students will recognize the relationship between genetics, the environment, and adaptation.
- Students will apply basic genetics concepts to individuals and to populations.
- Using gummy bears, students explore the basics of population genetics and the connections between genetics and natural selection.
- Students are also introduced to the concept of incomplete dominance. Students will choose two initial breeding pairs from their bags. Then, following the rules of gummy bear inheritance, the students will continue to choose and predict the offspring based on the characteristics of the original breeding pairs. This activity emphasizes the randomness inherent in genetics.
- Also, students will practice using Punnett squares and review phenotypes and genotypes.
- Brown paper bags.
- Red, orange, yellow, white, and green gummy bears with 4 bears of each color in each paper bag (1 set per group).
- Copies of worksheets (1 per group).
- Copies of gummy bear genetics chart (1 per group).
- This activity worked very well. The students practiced making Punnett squares. The students gained a much better understanding of the random nature of the sorting of alleles. They also began to see that genetics impacts not only the reproduction of individuals, but of populations and species. This activity seemed to make the genetics (which is a fairly abstract set of ideas) section more relevant to what they saw around them. It might be a good idea to have another snack for the students after the activity because the gummy bears used in the activity are handled and put onto the lab benches/desks.
- Engage: Ask students to make lists comparing and contrasting the fields of evolution and genetics. Discuss their lists as a class.
- Explore: Have students work in groups of 2 or 3 to complete all sections of the Gummy Bear Population Genetics activity except for the thought questions (complete questions 1 through 47 and leave out questions 48 through 52).
- Explain: Give students a short lecture, giving them examples of natural selection and explaining the mechanisms that create genetic variability within a population.
- Elaborate: Have students answer the thought questions of the Gummy Bear Population Genetics Activity (questions 48 through 52).
- Evaluate: Give students an example of environmental change that would select for some characteristics over others. Have students come up with an experiment to test the change of frequency in the morphs after the change as compared to the frequencies before the change.
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- Students need to be familiar with the following concepts before completing the activity (this may be accomplished through a short lecture prior to beginning the activity): 1) Dominant and recessive alleles, 2) Phenotypes and genotypes, 3) Incomplete dominance, 4) Punnett squares, 5) Adaptation, 6) Evolution through natural selection.
- This lesson will explore the relationship between evolution and genetics. It should be used after both topics have been covered (at least preliminarily). These two topics are fundamentally connected, but they are usually discussed and handled separately in biology classes.
- Evolution works through the process of natural selection. Natural selection is the process through which the traits that are most likely to help an organism survive and reproduce become more common in a given population. A well known example of natural selection is industrial melanism of the peppered moth. In England at the beginning of the Industrial Revolution, soot, dirt, and smoke darkened the trunks of trees and killed off the light colored lichens that grew on the tree trunks. The peppered moth, Biston betularia, was found in two morphs dark and light. The dark colored moth was rare prior to 1819. Through time, the dark colored moths (which were now much less visible to predators when the moths were sitting on the tree trunks) became much more common than they had previously been. There was a catch and recapture study conducted in 1950 that showed a large difference in the number of dark colored moths recaptured in industrial woods (those with darker tree trunks) and non-industrial woods (those with lighter tree trunks).
- Another example of natural selection is antibiotic resistance in bacteria. As antibiotics are used, most of the bacteria are killed. However, a small number of bacteria do survive. These surviving bacteria will reproduce, and their offspring are also likely to be resistant to the antibiotics. These differences are caused by chance mutations in the DNA of the bacteria. There are mutations in any population, most of which have no effect. However, when there is such a strong environmental driver, the few individuals who possess those mutations will survive and have a disproportionally large impact on the genetic make-up of future generations of the bacteria. Antibiotic resistance has become especially well known with regards methicillin-resistant Staphylococcus aureus (MRSA). Staph infections often affect people in close quarters, such as hospitals and prisons. As the use of antibiotics has increased, strains of staph that are resistant to many antibiotics, including penicillin and methicillin have emerged. These infections are incredibly difficult to treat because most antibiotics are ineffective in treating them.
- A third example of natural selection is Darwin’s finches. The Galapagos Islands are a chain of islands in the Pacific Ocean. Each island has different plants and animals that live there. There are finches found on most of those islands. The finches look very similar. The only visible difference between them is their beaks. Some have beaks that are better for cracking seeds while others have beaks that are better for getting insects out of the ground. The beaks correspond to the most abundant type of food on a given island. Islands with lots of available insects have finches with beaks that are really effective at eating the bugs.
- This relates back to genetics because natural selection needs something to select from. Sexual reproduction and the ability to pass traits from one generation to the next are what allow natural selection to work. The fact that through sexual reproduction, there is a recombination of the alleles of the parents (which were recombinations of their parents and so on) means that there are a variety of alleles within a population. If there was no variability, there can be no selection because there is nothing to select from.
- If there is no variation, there can be no natural selection. That is why sexual reproduction is such a good strategy. It leads to increased genetic diversity by randomly combining the alleles of the parents. Every generation creates new combinations of alleles.
- In order for one trait become “better” than another one, there has to be a reason. For example, in humans, there is no advantage or disadvantage to having brown eyes over having blue eyes. Brown eyes are more common because they are the dominant allele, but there is no natural selection for them. If the environment changed so that brown eyes were a disadvantage, blue eyes would become much more common. If there is no change in the environment or no some reason that one trait is more advantageous than another, there will be no pressure to remove that allele from the population. It will follow normal inheritance proportions (like those determined by the Hardy-Weinberg equilibrium principle) based on the dominance/recessive pattern for the alleles leading to that trait.
- The traits that are selected for will give the individuals that display those traits advantages in the form of increased survivorship and increased production of offspring. In other words, the individuals who have the “good” traits will survive to pass those traits along to their offspring. The ones with the “bad” traits won’t be able to pass their genes (traits) on as effectively.
- Eventually, unless the environment changes in a different way, the “bad” traits will become increasingly rare, and may disappear completely. The good traits will become more and more common. Though enough generations, not only may the physical trait be lost, but all genetic traces of a gene can be lost from a population (this takes a very long time, and may or may not ever happen completely).
- Divergence of populations can lead to speciation. If two (or more) populations become so genetically distinct that they can no longer produce viable offspring, they are considered to be two distinct species. This is generally driven by environmental changes that impact different populations in different ways.
- It is important to remember that the change in environment did not cause the change in the organisms. The variation had to exist within the gene pool of the population before the environment changed in order for it to appear in the later generations of the organism. However, because the advantage was so great, the organisms that carry the advantageous alleles have more offspring. These traits were selected by the environment, not created by it.
- The variation in populations comes from recombinations of genes as well as through random mutations in the genes. The mutations occur naturally. They can happen just because mistakes happen whenever something is done millions of times. It can also happen as a result of environmental factors (like UV radiation causing skin cancer). There are five main types of mutations: insertions, deletions, frame shifts, inversions, and point mutations.
- Insertions: Extra segments of DNA are inserted (can be one base or many bases) is inserted incorrectly into a DNA strand. Example: The cat ate the mat. Insertion: The cat ate the rat mat.
- Deletion: Segments of the DNA (one base or many bases) are deleted from the DNA strand. Example: The cat ate the mat. Deletion: The cat the mat.
- Frame Shifts: Pieces of DNA that aren’t in multiples of 3 are inserted or deleted into the strand. DNA is read in sets of 3 (called codons). For example, deleting 1 base will change the way that all the following sets of 3 are read. Example: The cat ate the mat. Frame Shift: The ata tet hem at.
- Inversions: A section of DNA is reversed. Example: The cat ate the mat. Inversion: The cat eht eta mat.
- Point Mutations: Point mutations occur when only one base is changed. This can cause problems (by changing the amino acid that a codon codes for) or not depending on where the change occurs and what the change is. Example: The cat ate the mat. Point Mutation: The cat tte the mat.
- To summarize, DNA mutations create new alleles within populations. The alleles are recombined through sexual reproduction, producing new traits. As environments change, different traits may become dangerous or advantageous. The parents with advantageous traits are more fit, so they have more offspring (some of which will carry the advantageous trait). Through many generations, the advantageous traits will become more and more common in the population.
- Kettlewell H.B.D. (1955). Selection experiments on industrial melanism in the Lepidoptera. Heredity 9:323-242.
- Kettlewell H.B.D. (1956). Further selection experiments on industrial melanism in the Lepidoptera. Heredity 10:287-301.
- Schito, G.C. (2006). The importance of the development of antibiotic resistance in Staphylococcus aureus. Clinical Microbiology and Infection 12 Suppl. 1: 3-8.
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Natural Selection
Genetic Variability in Population
Darwin's Finches
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- Time Frame: Designed to take up 55 minutes of a 90 minute period.
- Objective: Teach high school students about absolute plate motion, hotspot tracks and hotspot volcanoes by incorporating the use of data sets and up to date science to learn and understand scientific concepts.
- Biology Standard 2c: "Students know how random chromosome segregation explains the probability that a particular allele will be in a gamete."
- Biology Standard 2d: "Students know new combinations of alleles may be generated in a zygote through the fusion of male and female gametes (fertilization)."
- Biology Standard 2e: "Students know why approximately half of an individual’s DNA sequence comes from each parent."
- Biology Standard 2g: "Students know how to predict possible combinations of alleles in a zygote from the genetic makeup of the parents."
- Biology Standard 3a: "Students know how to predict the probable outcomes of phenotypes in a genetic cross from the genotypes of the parent and the mode of inheritance (autosomal or X-linked, dominant or recessive)."
- Biology Standard 7a: "Students know why natural selection acts on the phenotype rather than on the genotype of an organism."
- Biology Standard 7d: "Students know variation within a species increases the likelihood that at least some members of a species will survive under changed environmental conditions."
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