CC 3_19_24_Population Genetics and H-W.pptx

population genetics

1. Population Genetics Population : a group of individuals of the same species that live in the same area and interbreed to produce fertile offspring Gene pool : a population’s genetic makeup Consists of all copies of every type of allele If there is only one allele present for a particular locus in the population it is fixed Many fixed alleles=less genetic diversity

2. Gene Pool BB Bb bb BB B B B B B b b b

3. Population Genetics A population’s allele frequencies will change over time Remember: populations evolve NOT individuals Microevolution : small scale genetic changes in a population Evolution is driven by random occurrences Mutations Genetic Drift Migration/gene flow Natural selection

4. Mutations Review: define mutations in your own words Mutations can result in genetic variation Can form new alleles Natural selection can act on varied phenotypes Mutation rates tend to be slow in plants and animals and fast in prokaryotes due to a faster generation time Note : M utations can be harmful, neutral, or beneficial. Most mutations are in the neutral to harmful range. Not all mutations lead to evolution.

5. Genetic Drift Genetic drift : chance events that cause a change in allele frequency from one generation to the next Most significant to small populations Can lead to a loss of genetic variation Can cause harmful alleles to become fixed Does NOT produce adaptations Two types: Bottleneck effect Founder effect

6. Bottleneck Effect Bottleneck effect : when a large population is drastically reduced by a non-selective disaster Floods, famine, fires, hurricanes, hunting, etc. Some alleles may become overrepresented, underrepresented, or absent

7. 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 Lose genetic diversity

8. Gene Flow Gene flow : the transfer of alleles into or out of a population due to fertile individuals or gametes Alleles can be transferred between populations Example: pollen being blown to a new location

9. 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.

10. 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

11. 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

12. 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

13. 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

14. Natural Selection Now that you are familiar with natural selection, let’s look closer at how it can affect a population

15. Natural Selection Reproductive success is measured by relative fitness 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 There are three modes of natural selection Directional selection Stabilizing selection Disruptive selection

16. Modes of Natural Selection Selection towards one extreme phenotype Selection towards the mean and against the extreme phenotypes Selection against the mean. Both phenotypic extremes have the highest relative fitness

17. Natural Selection Sexual selection : a type of natural selection that explains why many species have unique/showy traits Males often have useless structures (ie colorful male peacock feathers) simply because females choose that trait Can produce traits that are harmful to survival Example: colorful feathers in male peacocks make them easier to spot by predators

18. Hardy Weinberg Equilibrium A model used to assess whether natural selection or other factors are causing evolution at a particular locus 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

19. Hardy Weinberg Equilibrium The Hardy Weinberg principle states: 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

20. Hardy Weinberg Equilibrium Five conditions must be met to be in Hardy Weinberg equilibrium: No mutations Random mating No natural selection Extremely large population size 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).

21. Think, Pair, Share Discuss with a partner how each of the five Hardy Weinberg conditions, if not met, can lead to evolution

22. Hardy Weinberg Equilibrium Two formulas: Frequency of the dominant allele in a population Frequency of the recessive allele in a population Percentage of the homozygous dominant individuals Percentage of the heterozygous individuals Percentage of the homozygous recessive individuals

23. Quick Check You are told that 20% of a population is homozygous recessive. What variable is that referring to in the Hardy Weinberg equations? It refers to q 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? It refers to p

24. 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 p 2 , 2pq and q 2 Most times you will use both formulas to complete the problem 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

25. Tips For Solving Problems Always write down both equations Identify the information given. Is it for alleles? For populations? Regardless of what the problem asks, solve for both p and q first , because that will allow you to answer any question! Use your calculator to do the square and square root functions! The most common mistake students make is assuming they know the answer. Double check your work

26. 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

27. 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? Given q 2 = 0.36 Remember: solve for p and q first regardless Work : 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%

28. 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? Work : p+q=1 p + 0.32 =1 p= 1- 0.32 p= 0.68 Answer : the frequency of homozygous dominant individuals is 46%. The frequency of heterozygous individuals is 44% Given q 2 = 0.10 p 2 =0.46 2pq: 2(.68)(.32)= 0.44 Check your work: p 2 + 2pq + q 2 = 1 0.46 +0.44 + 0.1 = 1 Solve for: 2pq and p 2

29. 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 : p+q=1 p + 0.26 =1 p= 1- 0.26 p= 0.74 p 2 =0.55 Answer : 55% of the population is homozygous dominant Given q 2 = 0.07 Solve for: p 2

30. 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? Work : p 2 = 123/1000= 0.123 q 2 = 422/1000 = 0.422 2pq= 455/1000 = 0.455 Answer : the frequency of the B allele is 35%, and b is 65% Given Uncalculated p 2 , 2pq, and q 2 Solve for: P and q p 2 = 0.123 p= q 2 = 0.422 q=

31. Quick Review What is the difference between genotype frequencies and allele frequencies in a population? 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). Hardy Weinberg equilibrium is often referred to as a null model of evolution. What does this mean? Answer : a null model of evolution is one where no evolution occurs, which is assumed with Hardy Weinberg if all conditions are met

32. Variations in Populations The more genetic diversity in a population, the better it can respond to changes in the environment 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