This preview shows page 1. Sign up to view the full content.
Unformatted text preview: Mutation and Genetic Drift Today ’s questions: I. How does mutation cause changes in allele frequencies? II. How does genetic drift cause changes in allele frequencies? I. How does mutation change allele frequencies? A. How often do mutations occur? Data from Caenorhabditis elegans (a roundworm). They are self‐fertile. Experimental design: Start with 1 individual; let it breed, let offspring mature in optimum conditions (food, moisture, no crowding) Choose 1 individual at random to be the parent of the next generation … let it breed, offspring grow, choose 1 at random to be the parent, etc. … repeat B. How do mutations affect fitness? Analyze the base sequence of the same genes at generations 1 and 250 Total of mutations per base per generation bases) (gametes are haploid; genome size is ~ 1. What is the average # of mutations/gamete? (~ genes/haploid genome) 2. What is average number of mutations/gene? Take‐home messages: In this population, see mutations change allele frequencies about /generation 3. How many generations would it take to create a 1% change in a particular gene? Important: By itself, mutation BUT There are new alleles in every gamete in every generation Important: Mutation constantly creates B. How do mutations affect fitness? Prediction: Data from the C. elegans experiment
Control lines (large populations, lots of competition, lots of selection) Experimental lines (tiny populations, no selection, mutations can accumulate) Time Lifespan II. II. How does genetic drift change allele frequencies? A. “Drift happens”: Chance variation in RS … random sampling error ... “luck” • In Viet Nam, U.S. soldiers boarded a helicopter and came under fire after lifting off. A guy got hit and killed. When the Grants were working on the Galápagos, they observed a small group of G. magnirostris arrive on Daphne Major and start a new population. When whites brought smallpox and measles to North America, up to 90% of Native Americans in some communities died. • • B. Experiment 1: Starting population size = 2 Mom: AHAT Dad: AHAT freq (AH) = freq (AT) = Kids (F1s) #1: #3: #5: #2: #4: #6: freq (AH) = freq (AT) = B. Experiment 2: Starting population size ~ 460 Moms: AHAT Dads: AHAT freq (AH) = freq (AT) = Flip a coin or use 2 little pieces of paper—one with “H” and one with “T”—to make your F1 offspring Clickers: 1. AHAH 2. AHAT 3. ATAT freq (AH) = freq (AT) = Punchlines: Is drift more important in small populations or large populations? What happens when drift continues, generation after generation (especially in small populations)? Consider the following experiment: 100 fruit fly populations in the lab, each consisting of 4 females and 4 males. In this starting population, alleles A1 and A2 are each at frequency 0.5. •Let them breed. Randomly pick 4 female and 4 male offspring to form the next generation in each population. Repeat for a total of 16 generations in each of the 100 populations. •Plot the number of populations (total of 100) that have allele A1 at various frequencies, from 0 (lost) to 1.0 (fixed). •Which of the following graphs predicts the result correctly? C. Consequences of genetic drift 1. With respect to fitness, drift … 2. With respect to genetic variation, drift … 3. Why is drift a concern for endangered species? ...
View Full Document