Week 2: Mechanisms of Evolution

(c) Kevin L. Callahan, University of Minnesota

Week 2: Mechanisms of Evolution NOTE: THERE WAS A HANDOUT GIVEN OUT IN LAB WHICH IS DUE THE FOLLOWING WEEK. This lab is designed to provide a few essential concepts about how the physical changes in a species through time can occur. Above all, this lab is intended to highlight the fact that evolution is NOT a process of improvement. Evolution is simply change in the characteristics of a population through time. If those changes create characteristics that help individuals in a population survive in a particular environment, then those characteristics are likely to become prevalent in succeeding generations. In today's lab you will examine two of the mechanisms by which evolution occurs, genetic drift and gene flow.

But first, some basic concepts: Read the chapters in your textbook, and watch for the following vocabulary terms: DNA adaptive allele chromosome co-dominant trait Darwinian fitness dominant trait founder effect gene genetic drift gene flow (migration) gene pool genotype heterozygous homozygous locus mutation natural selection phenotype population Punnett square recessive trait Note: Adaptive is not the same as adaptable. There is a Glossary Page at this website with a Vocabulary and Terms list for the course that has hypertext links to define the terms and connect you to websites or articles explaining the concept. Vocabulary: population a breeding group of organisms that tends to choose mates within the group gene the discrete unit of heredity allele genes whose locus is the same, but which are recognizably distinct (Alternate forms of a gene. There are usually two alleles per gene. If they are the same they are homozygous. If they are different they are heterozygous). locus the position of a particular gene and its alleles within the genome homozygous a zygote having two indistinguishable genes at a particular locus heterozygous a zygote having two recognizably different genes at a particular locus adaptive a feature or characteristic produced when a species responds and adjusts to changes in its environment (the functional response of organisms or populations to the environment) dominant trait one that is expressed phenotypically in both heterozygotes and homozygotes (A trait governed by an allele). recessive trait one that is expressed only in homozygotes co-dominant trait the situation where both alleles are expressed in the phenotype of heterozygotes (The expression of two alleles in heterozygotes). genotype the genes carried by an individual, or the combination of genes that an individual has at a particular locus phenotype an individual’s observable characteristics natural selection the force of evolution, clarified by Darwin, which acts deterministically through the differential fitness of genotypes in a population, to produce adaption chromosome long, thread-like structures consisting of DNA molecules and protein packaging (Discrete structures composed of DNA and protein found only in the nuclei of cells) gene pool the combined genes of a Mendelian population from which are drawn the genotypes of individual population members founder effect radical change in gene frequencies in an isolate, due to sampling error gene flow the movement of genes from one gene pool to another mutation a change in the genetic material, giving rise to a new allele in the population’s gene pool Darwinian fitness (of a genotype or phenotype) an individual’s success in transmitting its gene’s to the next generation, relative to that of others in the population (A measure of relative reproductive success of individuals measured by their genetic contribution to the next generation). Evolution the change in allele frequencies in a population over time (change in the characteristics of a population through time) genetic drift an undirected, random change of allele frequencies from generation to generation Punnett square A square broken into four quadrants that is used to chart out parents' possible offspring in order to predict genotype and phenotype Basic concepts: If the parents' genotype at a particular locus is known, we are able to determine the kinds of gametes that can be produced by those parents and, in turn, to predict the possible genotypes and phenotypes of their offspring. Three examples of problems that predict genotype and phenotype are offered below. All use a Punnett square to chart out the parents' possible offspring. Be sure you understand how this works. 1. Dominant/Recessive allele relationships. Consider the inheritance of freckles. The gene for freckles has two alleles: Label them F for freckles, and f for no freckles. F is dominant to f. If a woman is heterozygous for freckling and her husband has no freckles (is homozygous), what are the possible phenotypes and genotypes for their children? What % of the children will be heterozygous? What % will be homozygous? (Hint: The mother's genotype for freckles is Ff; the father's is ff.) Mother's gametes F f Possible Offspring: ------------------------ f Genotypes: 2 Ff; 2 ff; or ------------------------ 50% heterozygous and f 50% homozygous ------------------------ Phenotypes: freckles; no freckles 2) Co-dominant allele relationships: Consider the ABO blood group. There are three possible alleles for this locus, A, B, and O. A and B are co-dominant, meaning that when both alleles are present together, both are fully expressed (the blood type is AB). A and B are both dominant to O, and O is recessive. If a woman has genotype AO and a man has genotype AB, what are the genotypes possible for their children? Mother's gametes A O Possible Offspring: --------------------------- A Genotypes: AA, AO, AB, and BO --------------------------- B Phenotypes: A; AB; B --------------------------- 3) X-Linked inheritance: In people there are about 70 traits that are X-linked. Most are X-linked recessives like color blindness. Is it possible for a mother with normal sight to have a color- blind daughter? NOTE: Chromosome pair #23 is XX for females and XY for males. ONLY the X chromosome has the locus for the gene for color blindness and other X-linked traits! In this case, C is used for color sightedness, c is for color blindness; two symbols appear together because the allele C or c can only appear on the X chromosome, not on the Y. Mother's gametes XC Xc Although XC, XC would be a normal-sighted ------------------- female, this genotype is not possible here. Xc ------------------- Possible Genotypes and Phenotypes: Y0 XC, Xc = normal-sighted female ------------------- Xc, Xc = color-blind female XC, Y0 = normal-sighted male Xc, Y0 = color-blind male As shown above, it is possible for a mother with normal sight to h ave a color-blind daughter-- if the mother were heterozygous for color-blindness and the father were color blind. ---------------------------------------------------------------------------------------------------------------- Genetic drift is an undirected, random change of allele frequencies in a population from generation to generation. In any population that reproduces itself sexually, alleles are shuffled randomly each generation by the process of gamete formation. However, since the possible alleles in each generation can only be drawn from the alleles that are present in the parent generation, one would expect to get the same ratio of alleles in each new generation. Generally this expectation holds true for large populations, where the number of recombinations of alleles (matings) in each generation is high enough to reach the statistical probability of matching the frequencies in the parent generation. The explanation of this statistical probability is the same as that for flipping a coin. If you flip a quarter only 10 times, you might get only 1 heads, and 9 tails. However, if you flip it 1000 times, chances are that you will get approximately 500 heads and 500 tails (a 50:50 or 1:1 ratio of frequency). If you have a small population reproducing, you are, so to speak, limiting the number of times you flip the coin. The ratio between two alleles can shift dramatically from one generation to the next in a small population: this shift in frequency is genetic drift. Genetic drift can begin a trend that results in one allele vanishing completely from the gene pool of a small group, thus decreasing or eliminating variation in that population with regard to that gene. But, since the change is undirected--nothing more than random chance is causing an allele to increase or decrease in frequency--the changes of allele frequencies in several different small groups of a species are likely to be different. Genetic drift is therefore likely to increase variation between groups, but to decrease variation within them. Remember that our earliest hominid ancestors probably lived in small groups. Gene flow is the exchange of genetic information between populations, or, more specifically, the movement of genes from one gene pool to another. Frequently this process occurs when individuals move from one group to another. ---------------------------------------------------------------------------------------------------------------- Evolution Exercise You will facilitate the birth of five generations of a small population in conditions that favor genetic drift. Then you will create another three generations in conditions where gene flow occurs. For each generation, you will track the changing ratio between two alleles of a gene. Equipment: 1. a paper bag holding 10 colored and 10 clear marbles 2. a plastic bag holding replacement marbles Check the ratio in your paper bag before you begin. This bag holds the gene pool for your parent population of 10 individuals. The colored marbles are X alleles, the clear marbles are O alleles. Procedure: Note: Every member of your group should make his/her own record of each generation on the data sheets provided. Part I: Genetic Drift 1. To create the first individual of the next generation, reach into the paper bag and, without looking, draw two marbles. These marbles represent the genotype for the new zygote. 2. Record the genotype of this individual on the data sheet. 3. Return the marbles to the bag. 4. Repeat this process until you have created a total of 10 individuals. 5. When all 10 individuals have been born, record the total number of X alleles and the total number of O alleles in this new generation. Check to make sure they add up to 20 alleles. 6. Before you begin the birth of the third generation, you must change the gene pool (the marbles in your bag) to reflect the allele frequencies in the new generation (those now having children). Adjust the number of colored and clear marbles to reflect the X:O ratio of the second generation. 7. Continue for 5 generations. 8. Record the X:O final ratio of each OTHER group in your lab below. 9. Discussion questions: a. Can you see any trends in the changes of your group's population from generation to generation? What is happening to the variation in your group's population? b. What do you think would happen if you drew marbles from the bag 1000 times per generation instead of only 10? Why? c. What would happen if you continued this exercise for 50 generations instead of 5? Part II: Gene Flow 1. Trade your fifth-generation gene pool with a neighboring group. Draw 3 individuals (3 PAIRS of marbles) from the borrowed gene pool, keeping each pair out of the bag after they are drawn. 2. Retrieve your own gene pool and add the new marbles to it. 3. Record the new ratio of X:O and produce the next generation as before. 4. Adjust the number of clear and colored marbles in the bag to reflect the gene pool of your second generation. 5. Repeat steps 1-4 for two more generations. 6. Record the final X:O ratio of each OTHER group in your lab below. 7. Discussion questions: a. What trends do you notice here? How do they differ from those of the genetic drift section? b. What has happened to the variation in your group's gene pool? In the gene pools of the other groups in your lab? NOTE: THERE WAS A HANDOUT GIVEN OUT IN LAB WHICH IS THE ASSIGNMENT DUE THE FOLLOWING WEEK.

Links to other sites on the Web

The Human Genome Project
A Glossary of Molecular Genetics
A Primer on Molecular Genetics
DNA, Genes & Chromosomes
Populus (freeware from the U of M (EEB) for simulating genetic drift and gene flow etc.)

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