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in your own words • Mutations can result in genetic variation o Can form new alleles o Natural selection can act on varied phenotypes o Mutation rates tend to be slow in plants and animals and fast in prokaryotes due to a faster generation time Note: Mutations can be harmful, neutral, or beneficial. Most mutations are in the neutral to harmful range. Not all mutations lead to evolution. © Getting Down with Science Genetic Drift https://www.youtube.com/watch?v=WOTM4LQmoZY • • Genetic drift: chance events that cause a change in allele frequency from one generation to the next. Some individuals with specific allele reproduce more than the others O Most significant to small populations o Can lead to a loss of genetic variation o Can cause harmful alleles to become fixed o Does NOT produce adaptations o Two types: Bottleneck effect ■ Founder effect © Getting Down with Science GENETIC DRIFT CHANGE IN ALLELE FREQUENCY DUE TO A CHANCE EVENT SWAT! RATIO 5:5 RATIO 5:51 RATIO 10:2 REPRODUCTION Bottleneck Effect Bottleneck effect: when a large population is drastically reduced by a non-selective disaster o Floods, famine, fires, overrepresented, underrepresented, or Original population hurricanes, hunting, etc. event o Some alleles may become absent Bottlenecking Surviving population © Getting Down with Science ORIGINAL POPULATION BOTTLENECK EFFECT BOTTLENECK EVENT SURVIVING POPULATION NEW POPULATION Founder Effect Founder effect: when a few individuals become isolated from a large population and establish a new small population with a gene pool that differs from the large population o Lose genetic diversity Parent population © Getting Down with Science Migrating population Population after a couple of generations Founder population after a few more generations Gene Flow • If genetic variants are carried to a population where they previously did not exist, gene flow can be an important source of genetic variation. In the graphic below, a beetle carries the gene version for brown coloration from one population to another. • Ex. Plant populations experience gene flow by spreading their pollen long distances. Quick Review Examine each scenario and determine if it is an example of the bottleneck effect, founder effect, or genetic drift. Justify each response with an explanation. © Getting Down with Science Scenario 1 The black robin is a small bird native to the Chatham Islands in New Zealand. Before the 19th century, much of their habitat was converted to farmland. Due to the loss of their habitat and the introduction of non-native predators (like cats), the population declined to its lowest point, only 5 individuals. Since then, conservation efforts have brought the population number back up to an estimated 230 individuals. Bottleneck effect due to human activity © Getting Down with Science Scenario 2 In the 1720's an Amish population settled into Pennsylvania. A small number of the early settlers were from Germany and carried an unusual number of mutations, one of which included polydactyly. The trait for polydactyly is now much more common in the Amish population than it is in the general population. Founder effect Scenario 3 Recurrent flash floods have caused mass mortalities in the marble trout population. Scientists have analyzed the genetic makeup of the remaining marble trout population and have found that genetic diversity has decreased significantly. Bottleneck effect © Getting Down with Science Scenario 4 Due to an increase in wind in a local area, pollen from one population of flowers has travelled further than usual and pollinated another population of flowers. The offspring of these flowers now have traits of each population. Gene flow © Getting Down with Science Natural Selection Now that you are familiar with natural selection, let's look closer at how it can affect a population © Getting Down with Science Natural Selection • Reproductive success is measured by relative fitness o The number of surviving offspring that an individual produces compared to the number left by others in the population • Effects of natural selection can be measured by examining the changes in the mean of phenotypes o There are three modes of natural selection 1. Directional selection 2. Stabilizing selection 3. Disruptive selection © Getting Down with Science Modes of Natural Selection After selection Before selection Directional selection Selection towards one extreme phenotype https://www.youtube.com/watch?v=WmTlwD2Zd7E&t=507s Stabilizing selection Selection towards the mean and against the extreme phenotypes © Getting Down with Science Disruptive selection a Selection against the mean. Both phenotypic extremes have the highest relative fitness Natural Selection Sexual selection: a type of natural selection that explains why many species have unique/showy traits o Males often have useless structures (ie colorful male peacock feathers) simply because females choose that trait o Can produce traits that are harmful to survival ■ Example: colorful feathers in male peacocks make them easier to spot by predators Now do page 26 in your packet © Getting Down with Science Hardy Weinberg Equilibrium • A model used to assess whether natural selection or other factors are causing evolution at a particular locus o Determines what the genetic make up of the population would be if it were NOT evolving ■ This is then compared to actual data: • If there are NO differences, then the population is NOT evolving If there ARE differences, then the population MAY BE evolving © Getting Down with Science Hardy Weinberg Equilibrium • The Hardy Weinberg principle states: o The frequencies of alleles and genotypes in a population will remain constant from generation to generation, provided that only Mendelian segregation and recombination of alleles are at work Remember: this is a hypothetical situation where no evolution would take place. In real populations the allele and genotype frequencies DO change over time © Getting Down with Science Hardy Weinberg Equilibrium Five conditions must be met to be in Hardy Weinberg equilibrium: 1. No mutations 2. Random mating 3. No natural selection 4. Extremely large population size 5. No gene flow • If any of these conditions are not met, then microevolution occurs (i.e. mutation, gene flow, genetic drift, natural selection, and non-random mating). © Getting Down with Science Hardy Weinberg Equilibrium Two formulas: p+q=1 Frequency of the dominant allele in a population Percentage of the homozygous dominant individuals p²+2pq+q² = 1 Frequency of the recessive allele in a population Percentage of the homozygous recessive individuals Percentage of the heterozygous individuals © Getting Down with Science Quick Check 1. You are told that 20% of a population is homozygous recessive. What variable is that referring to in the Hardy Weinberg equations? a. It refers to q² 2. You are told that the frequency of a dominant allele in a population is 75%. What variable is that referring to in the Hardy Weinberg equations? a. It refers to p © Getting Down with Science Hardy Weinberg Equilibrium Which formula you start with depends on the information you are given. • If a problem gives "allele frequencies," it is referring to "p" and "q." If it gives information about individual organisms or populations, then it is referring to p², 2pq and q² o Most times you will use both formulas to complete the problem o Usually you are given q, and then you will need to find p, but you will also see problems where that is not the case © Getting Down with Science Tips For Solving Problems 1. Always write down both equations 2. Identify the information given. Is it for alleles? For populations? 3. Regardless of what the problem asks, solve for both p and q first, because that will allow you to answer any question! 4. Use your calculator to do the square and square root functions! The most common mistake students make is assuming they know the answer. 5. Double check your work © Getting Down with Science Hardy Weinberg Let's watch a video by Mr. Anderson and see some Hardy Weinberg problems being solved, then we will practice some problems on our own! https://www.youtube.com/watch?v=xPkOAnK20kw&t =571s © Getting Down with Science Practice Problems 1. You have sampled a population and found that 36% are homozygous recessive (aa). What is the frequency of the A and a alleles? p+q = 1 p²+2pq+q² = 1 Given q²= 0.36 Remember: solve for p and q first regardless Work: √q² = 0.6 p+q=1 p+ 0.6=1 p= 1-0.6 p= 0.4 Answer: the frequency of the A allele is 40%. The frequency of the a allele is 60% © Getting Down with Science Practice Problems 2. Ten percent of a population of mice are albino. Albinism is recessive to tan coloring. What is the frequency of homozygous dominant and heterozygous individuals in the population? p+q=1 p²+2pq+q² = 1 Given q² = 0.10 Solve for: 2pq and p² Work: √q² = 0.32 p+q=1 p + 0.32 =1 p= 1-0.32 p= 0.68 p²=0.46 2pq: 2(.68)(.32)= 0.44 Check your work: p² + 2pq+q² = 1 0.46 +0.44 +0.1 = 1 Answer: the frequency of homozygous dominant individuals is 46%. The frequency of heterozygous individuals is 44% © Getting Down with Science Practice Problem #3 3. A recessive genetic disorder occurs in 7% of a population of birds. What percentage of the population is homozygous for the normal condition? Work: √q² = 0.26 p+q=1 p + 0.26 =1 p+q=1 p²+2pq+q² = 1 Given q²= 0.07 Solve for: p= 1-0.26 p= 0.74 p²=0.55 p² Answer: 55% of the population is homozygous dominant © Getting Down with Science Practice Problem #4 4. A population has 1000 individuals. 123 BB, 455 Bb and 422 bb. What are the frequencies of the B and b allele? p+q = 1 p²+2pq+q² = 1 Given Uncalculated p², 2pq, and q² Solve for: P and q Work: p²= 123/1000= 0.123 q²= 422/1000 = 0.422 2pq= 455/1000 = 0.455 p²= 0.123 p=√0.123 -0.35 q²= 0.422 q= √0.422=0.65 Answer: the frequency of the B allele is 35%, and b is 65% © Getting Down with Science Quick Review 1. What is the difference between genotype frequencies and allele frequencies in a population? a. Answer: genotype frequencies refer to how alleles combine. Allele frequencies refer to an allele's relative distribution in the population (ie how common is the allele). 2. Hardy Weinberg equilibrium is often referred to as a null model of evolution. What does this mean? a. Answer: a null model of evolution is one where no evolution occurs, which is assumed with Hardy Weinberg if all conditions are met © Getting Down with Science Variations in Populations • The more genetic diversity in a population, the better it can respond to changes in the environment o More likely to be individuals that can withstand changes ■ Species with low genetic diversity are at risk of decline and/or extinction • Example: due to poaching and poisoning, the California Condor was reduced to 27 individuals. This drastically lowered the gene pool. Even though restoration efforts have increased numbers, they have lost diversity © Getting Down with Science