chapter 22: descent with modification chapter 25.2 & 3 & 4 : history of life on earth

1. Unit 1: Evolution chapter 22: descent with modification chapter 25.2 & 3 & 4 : history of life on earth chapter 23: evolution of populations chapter 24 and 25.6 : origin of species chapter 26.1 and 26.3: phylogeny

2. Chapter 23: Evolution of Populations Individual birds do not change in response to changes in the environment. Populations evolve! (microevolution = change in genetic frequencies in a population over time)

3. The proportion of the population that possess a beneficial, heritable adaptive variant increases over time due to natural selection. • Individuals of all species show variation. • Some variation is geographic. • Some variation is heritable; some is not. • For evolution to occur, there must be genetic (heritable) variation. nonheritable variation (differences in appearance due to diet, i.e., acquired characteristics) heritable variation

4. How does genetic variation arise in the first place to allow for natural selection?Sources of genetic variation • formation of new alleles • New alleles arise by mutation. • Mutations in germ cells (forming gametes) are passed to offspring. • Most mutations are repaired or harmless (or happen in somatic cells). • Some mutations cause harm, and some are beneficial. Context is crucial! • altering gene number • Errors in meiosis can duplicate genes and even portions of chromosomes. • Expanded genomes have played a major role in evolution. • sexual reproduction • Crossing over and independent assortment during meiosis create gametes with unique genetic combinations. • Fertilization during sexual reproduction provides fresh combinations . • Source of much of the genetic variation for evolution Does the mere presence of genetic variation in a population guarantee that evolution will occur?

5. How do we know if a population is evolving? In order to answer this question, we will use the following vocabulary… • population = a group of individuals of the same species in the same area which interbreed and produce fertile offspring • gene = a unit of hereditary information (DNA) for a given trait • alleles = alternative versions of a gene which produce different physical effects • locus = location on a chromosome where a gene is found • gene pool = all copies of every allele at every locus for a population • genotype = genetic makeup (set of alleles) for an organism • homozygous = two of the same alleles for a given gene • heterozygous = two different alleles for a given gene • phenotype = observable physical traits of an organism (physical manifestation of genotype)

6. Example… • population of 500 wildflowers • two alleles for gene that controls flower color (CRand CW); exhibit incomplete dominance • CRCR = red flowers (320 flowers) CRCW = pink flowers (160 flowers) CWCW = white flowers (20 flowers) In this wildflower population… • How many CR alleles are in the gene pool? • How many CW alleles are in the gene pool? • What are the frequencies of these two alleles in the gene pool?

7. Hardy-Weinberg Principle • Frequencies of alleles and genotypes will remain constantfrom generation to generation if reproduction is a completely random process and if no evolution is occurring in the population. • Assuming one gene with two alleles (the simplest case scenario)… p = frequency of one allele (usually the dominant allele) q = frequency of other allele (usually the recessive allele) p + q = 1 WHY? • Using the example from the previous slide: gene pool

8. Hardy-Weinberg Equilibrium (HWE) • Assuming a population is not evolving, we can expect the genotypes to be in equilibrium (as predicted by the HW equation below). • HWE predicts that at a gene locus with two alleles, the three possible genotypes will appear in the following proportions (or frequencies) from generation to generation to generation: p2 = frequency of individuals homozygous for p allele 2pq = frequency of heterozygous individuals q2 = frequency of individuals homozygous for q allele p2 + 2pq + q2 = 1 WHY?

9. The Hardy-Weinberg equation is often used as an initial test of whether evolution is occurring in a population…

10. A population has 700 individuals, 85 of genotype AA, 320 of genotype Aa, and 295 of genotype aa. What are the frequencies of alleles A and a? • The frequency of allele a is 0.45 for a population in Hardy-Weinberg equilibrium. What are the expected frequencies of genotypes AA, Aa, and aa? • In fruit flies, the red eye allele (A) is dominant to the sepia (brown) allele (a). You have 1000 fruit flies, and 640 of them have red eyes. The remaining flies have sepia eyes. Assuming the population is in Hardy-Weinberg equilibrium, how many individuals would you expect to be homozygous for red eye color? • A locus that affects susceptibility to a degenerative brain disease has two alleles, A and a. In a population, 16 people have genotype AA, 92 have genotype Aa, and 12 have genotype aa. Is this population in Hardy-Weinberg equilibrium? Explain.

11. Scientists investigating HIV infection have identified a co-receptor protein (CCR-5) on the surface of cells that HIV particles must bind to in order to establish an infection. Two alleles of the CCR-5 gene have been identified in a certain population: CCR-5 (dominant, wild-type allele) and Δccr-5 (recessive, null allele). In this population, 704 individuals were genotyped with respect to their CCR-5 alleles, and the following information was obtained: CCR-5 homozygotes: 582 CCR-5/Δccr-5 heterozygotes: 114 Δccr-5 homozygotes: 8 Find the allele frequencies for each CCR-5 allele in this population and indicate whether or not they are in Hardy-Weinberg equilibrium. Data taken from: Samson, et al. Nature. (1996) 382:722

12. Hemoglobin is the oxygen-carrying molecule in red blood cells. In this simplest scenario of hemoglobin genetics, there are two alleles: A (codes for fully functional hemoglobin) and S (codes for a hemoglobin molecule that folds into the wrong shape and is nonfunctional). A is dominant to S. Individuals that are homozygous for the S allele have the genetic condition sickle cell anemia. In a certain Nigerian population, 12,387 individuals were genotyped for their hemoglobin alleles. The following data was recorded: AA homozygotes: 9365 AS heterozygotes: 2993 SS homozygotes: 29 Find the allele frequencies for each hemoglobin allele (A and S) in this population and indicate whether or not the population is in Hardy-Weinberg equilibrium for this gene. Data taken from: Zimmer and Emlen. Evolution: Making Sense of Life. (2013) Roberts and Company Publishers, Inc.; Stone et al. Genes, Culture, and Human Evolution: A Synthesis. (2007) Wiley-Blackwell

13. Conditions Required for Hardy-Weinberg Equilibrium to Exist: • no mutations • random mating • no natural selection • extremely large population size • no gene flow • If a population is not in HW equilibrium, at least one of the conditions above is not being met. • Real populations change over time (evolve)! • Real populations are often not in HW equilibrium (with respect to everyallele of everygene).

14. Causes of Genetic Frequency Changes (Microevolution) • New mutations • Rare (relatively speaking); why? • If mutation alters fitness, this can greatly change allele frequency. Will the frequency increase or decrease? • Nonrandom mating (selective picking of a mate) • Sexual selection • Natural selection • differential success in survival and reproduction • Selection results in alleles being passed to the next generation in proportions that differ from those in the present generation. • leads to adaptive evolution • Genetic drift • chance events • Gene flow • movement into or out of population

15. Genetic drift • founder effect • e.g., Amish populations • bottleneck effect • e.g., cheetah, bison, mitochondrial Eve • significant in small populations • can lead to loss of genetic variation • harmful alleles can become fixed • Gene flow • transfer of alleles into or out of a population due to movement of individuals and/or gametes (immigration vs. emigration) • reduces genetic variation between populations • increasingly important agent of evolutionary change in modern human populations; why?

16. A closer look at natural selection… • Selection acts directly on phenotype. • Selection acts indirectly on genotype. • Natural selection is the only mechanism that consistently causes adaptive evolution! • In a given environment, some traits lead to greater relative fitness.

17. Finch beak size increases with increases in seed size. Artificial selection (dogs, cattle, pigeons…)

19. Ok to eat (palatable) Toxic Toxic Toxic Different species Same species African swallowtails – females mimic harmful species with extreme phenotypes; birds eat intermediates.

20. An Australian Aboriginal birth cohort: a unique resource for a life course study of an Indigenous population. A study protocol Susan M Sayers, Dorothy Mackerras, Gurmeet Singh, Ingrid Bucens, Kathryn Flynn and Alison Reid human birth weight – selection for intermediate size (6.6 – 8.8 lbs) plant height – tall plants damaged by wind, short plants can’t compete for sunlight

21. Sexual Selection (nonrandom mating) peacock and peahen • Individuals with certain characteristics are more likely to obtain mates. • leads to sexual dimorphism • intrasexual selection: competition within same sex (usually males compete for females) • intersexual selection: mate choice, usually on the part of the females widowbird antlers

22. Directional and stabilizing selection reduce variation. So how is genetic variation preserved in a population? (Why would preserving genetic variation be a good thing?) A. diploidy recessive alleles can remain phenotypically hidden in heterozygotes (carriers) B. balancing selection: selection which maintains two or more forms in a population. Includes: 1. heterozygote advantage - Heterozygotes have greater fitness than either homozygous condition.

24. Grigor, L., et al. PNAS. (2001) 98:6253 selection favors whichever mouth phenotype is least common 2. Frequency-dependent selection Fitness of phenotype depends upon how common it is in population selection favors whichever color phenotype is least common