5 FINGERS OF EVOLUTION

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1 MICROEVOLUTION Student Packet SUMMARY EVOLUTION IS A CHANGE IN THE GENETIC MAKEUP OF A POPULATION OVER TIME Microevolution refers to changes in allele frequencies in a population over time. NATURAL SELECTION IS A MAJOR MECHANISMS OF EVOLUTION Natural selection increases the frequency of beneficial alleles that provide reproductive advantage. Adaptation refers to a trait that evolves through natural selection. Environments may be stable or fluctuating and affect evolutionary rate and direction. Natural selection acts on phenotypes, not genotypes. Different phenotypes may be favored in different environments. Natural selection acts on individuals, but only populations evolve. Types of natural selection include directional, disruptive and stabilizing selection. EVOLUTION IS ALSO DRIVEN BY GENE FLOW, MUTATIONS, NON-RANDOM MATING AND GENETIC DRIFT Gene flow results from migration of individuals. Mutations may increase genetic variation by producing new alleles. Non-random mating may result in changes in genotype frequencies in a population. Genetic drift is the random loss of individuals and their alleles due to bottleneck effect (only a few individuals survive a random event) or founder effect (a small number of individuals colonize a new region). In small populations, genetic drift may greatly reduce genetic variation. Natural Selection Gene Flow Mutations 5 FINGERS OF EVOLUTION Non-random Mating Genetic Drift 1

2 EVOLUTION CAN BE MEASURED BY CHANGES IN ALLELE FREQUENCIES Hardy-Weinberg equilibrium equations predict genotype frequencies from allele frequencies in the absence of evolution (no natural selection, no gene flow, no mutations, random mating and no selection). p + q = 1 p 2 + 2pq + q 2 = 1 If p and q change over time, evolution is taking place. p = frequency of the dominant allele (A) q = frequency of the recessive allele (a) p 2 = frequency of the homozygous dominant genotype (AA) q 2 = frequency of the homozygous recessive genotype (aa) 2pq = frequency of the heterozygous genotype (Aa) SEVERAL PROCESSES INCREASE GENETIC VARIATION Phenotypic variations are not directed by the environment but occur by random changes: 1. Mutations increase genetic variation due to the imperfect nature of DNA replication and repair. 2. Sexual reproduction increases genetic variation in eukaryotes through the following processes: crossingover in meiosis, the independent assortment of chromosomes in meiosis and random fertilization. 3. The horizontal acquisitions of genetic information primarily in prokaryotes increases genetic variation via several processes: transformation (uptake of naked DNA), transduction (viral transmission of genetic information), conjugation (cell-to-cell transfer) and transposition (movement of DNA segments within and between DNA molecules). HUMANS IMPACT VARIATION IN OTHER SPECIES Throughout the history, humans have selected for certain desirable traits. Artificial selection occurs when humans manipulate a gene pool. There are often consequences involved in such manipulations. For example, farmers try to increase crop production, which may lead to many farmers growing only one variety of a particular crop such as corn. This leads to a loss of genetic diversity. If a disease attacks that particular variety of corn, the farmers growing that variety lose their entire crop. Other examples of consequences of artificial selection include antibiotic resistant bacteria, herbicide resistant weeds and insecticide resistant insects. GENETIC VARIATION IN A GENE POOL IS BENEFICIAL A population s ability to respond to changes in the environment is affected by genetic diversity. Genetic variation provides cells, organisms and populations with a wider range of functions. For example, not all individuals in a population in a disease outbreak are equally affected; some may not show symptoms, some may have mild symptoms and some may be naturally immune to the disease. Species and populations with little genetic diversity are at risk for extinction as seen in the examples of prairie chickens and potato blight. One mechanism that helps maintain genetic variation is the heterozygote advantage. Heterozygote advantage involves an individual who is heterozygous (a.k.a. carrier ) and has a greater fitness than a homozygous individual under a particular condition. For example, if a person inherits the sickle cell gene from one parent and a normal hemoglobin gene from the other parent, he/she has a normal life expectancy and is also resistant to the malarial parasite. 2

3 MULTIPLE CHOICE QUESTIONS 1. Road construction has isolated a small portion of a beetle population from the main population. After a few generations, this new population exhibits dramatic genetic differences from the original population. These genetic differences are most likely because a. mutations are more common in the new environment. b. allele frequencies among the stranded beetles differed by chance from those in the parent population s gene pool and subsequent genetic drift caused even more divergence from the original gene pool. c. gene flow increases in the new environment. d. the new environment is different from the old, favoring directional selection. 2. The pesticide DDT was widely used as an insecticide prior to 1950s. The table below shows the time frame for the development of DDT resistance in mosquitoes. Which of the following best explains the increase in the percentage of mosquitoes resistant to DDT over time? a. Migration of mosquitoes from other areas leads to gene flow between different populations and increases the frequency of the resistant phenotype. b. Natural selection causes the mosquitoes to increase their mutation rate and adapt to the presence of DDT. c. Competition for limited resources causes mosquitoes to migrate to geographical areas that were sprayed with the insecticide DDT. d. Selective pressures favor mosquitoes with the resistant phenotype, causing the frequency of the resistant allele in the population to increase. 3. Given a population that contains genetic variation, what is the correct sequence of the following events, under the influence of natural selection? 1. Differential reproductive success takes place. 2. A new selective pressure occurs within the environment. 3. Allele frequencies within the population change over time. 4. Environmental change occurs within an area. a. 2, 4, 1, 3 b. 4, 1, 2, 3 c. 4, 2, 1, 3 d. 4, 2, 3, 1 3

4 Questions Black-footed ferrets are carnivorous mammals that feed mostly on prairie dogs and are native species of central North America. Black-footed ferrets are listed as endangered because today they occupy only about 2% of their original range. The species declined in the 20 th century due to decrease in prairie dog numbers by habitat destruction. The trends in the black-footed ferrets population size and their amount of genetic diversity are shown in the graph below. Genetic diversity is measured by the number of alleles per locus, proportion of polymorphic loci (loci with 2 or more alleles) or by the frequency of heterozygotes per locus. Locus refers to a location of a gene on a chromosome. N = number of black-footed ferrets A 0 (triangles) = number of alleles per locus P (diamonds) = proportion of polymorphic loci He (squares) = frequency of heterozygotes per locus Which of the following is the most likely consequence of the decrease in genetic diversity? a. Black-footed ferrets are at risk for extinction because a population s ability to respond to changes in the environment is affected by genetic diversity. b. Random mating in the remaining black-footed ferrets will restore the original genetic diversity of the population. c. Changes in gene frequencies resulting from the genetic drift will not have a measurable impact on the small population of black-footed ferrets. d. Decreased genetic diversity will allow individual black-footed ferrets in the population to respond more favorably to changes in environmental conditions. 5. Which of the following is the best explanation for why high frequency of heterozygotes is correlated with higher genetic diversity? a. A heterozygote has a more advantageous phenotype than homozygotes. b. In the heterozygous condition, the recessive allele is hidden from natural selection, allowing the variation to be maintained for future generations. c. Having a high proportion of heterozygotes in a population minimizes the chances of offspring being affected by recessive deleterious mutations. d. A heterozygote individual experiences a higher reproductive success, passing its genes to offspring and preserving them for future generations. 4

5 6. Researchers Koehn and Hilbish studied genetic variation in the marine mussel Mytilus edulis around Long Island, New York. The researchers measured the frequency of the lap 94 allele that codes for a protein involved in regulation of internal water balance. Which of the following is the best explanation for the differences in the lap 94 allele frequencies? a. Mussels with high frequency of the lap 94 allele are selected against in high salinity areas. b. Mussels living in areas with lower salinity do not need to maintain osmotic homeostasis. c. Mussels with the lap 94 allele are able to maintain osmotic balance in water with high salt concentration. d. Higher frequencies of the lap 94 allele increase genetic variation in mussels living in areas with high salt concentration. 7. Black-bellied seedcracker finches feed on seeds of various marsh plants in Cameroon. The small-beaked finches feed mainly on soft seeds while the large-billed finches specialize in cracking hard seeds. The graph shows the current distribution of beak sizes in the population of black-bellied seedcracker finches. If the marsh plants with soft seeds become scarce in the future, what change will most likely occur in the beak sizes of the blackbellied finch population? a. The proportion of birds with intermediate-sized beaks will increase due to stabilizing selection. b. The proportion of large-billed finches will increase due to directional selection. c. The beak range will become wider due to disruptive selection. d. The proportion of large-billed, intermediate-billed, and small-billed finches will stay the same because the decrease in soft seed availability will not affect beak sizes. 5

6 MATH GRID IN 1. In humans, there exists a rare condition known as polydactyly, characterized by extra fingers or toes. This autosomal dominant genetic condition is caused by a base pair mutation within a hox gene associated with embryonic development of the limb buds and has an estimated prevalence of 34 out of 100,000 individuals. Calculate the frequency of the dominant allele to the nearest ten thousandth. SHORT FREE RESPONSE QUESTIONS 1. A research team has genetically engineered a strain of fruit flies to eliminate errors during DNA replication. The team claims that this will eliminate genetic variation in the engineered flies. A second research team claims that eliminating errors during DNA replication will not entirely eliminate genetic variation in the engineered flies. a. Provide ONE piece of evidence that would indicate new genetic variation has occurred in the engineered flies. b. Describe ONE mechanism that could lead to genetic variation in the engineered strain of flies. c. Describe how genetic variation in a population contributes to the process of evolution in the population. 6

7 2. A group of botanists studied variation in leaf mass among tropical trees B. forficata grown in their natural conditions in forest gaps and understory. The data table summarizes the results for 25 trees in each experimental group. STATISTICAL ANALYSIS LEAF MASS PER AREA (mg/cm 2 ) Forest Gaps Understory Mean 4 2 Standard Deviation Standard Error of the Mean.25.5 a. Graph the means of leaf mass per area for B. forficata trees grown in forest gaps and understory to within 95 percent confidence (sample mean ±2 SEM). b. Based on the sample means and standard errors of the means, determine whether there are statistically significant differences in B. forficata trees grown in forest gaps and understory. Justify your response. 7

8 LONG FREE RESPONSE QUESTION Evolution is one of the unifying themes of biology. Evolution involves change in the frequencies of alleles in a population. For a particular genetic locus in a population, the frequency of the recessive allele (a) is 0.4 and the frequency of the dominant allele (A) is 0.6. a. Calculate the frequency of each genotype (AA, Aa, aa) in this population and calculate the frequency of the dominant phenotype. b. Discuss how the Hardy-Weinberg principle of genetic equilibrium can be used to determine whether this population is evolving. c. Identify a particular environmental change and describe how it might alter allelic frequencies in this population. Explain which condition of the Hardy-Weinberg principle would not be met. 8