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The Evolution of Populations Chapter 23 What you must know: How mutation and sexual reproduction each produce genetic variation. The conditions for Hardy-Weinberg equilibrium. How to use the Hardy-Weinburg equation to calculate allelic frequencies and to test whether a population is evolving. What effects genetic drift, migration, or selection may have on a population, and analyze data to justify your predictions. Smallest unit of evolution Microevolution: change in the allele frequencies of a population over generations Darwin did not know how organisms passed traits to offspring 1866 - Mendel published his paper on genetics Mendelian genetics supports Darwin’s theory Evolution is based on genetic variation Review: Sources of Genetic Variation Point mutations: changes in one base (eg. sickle cell) Chromosomal mutations: delete, duplicate, disrupt, rearrange usually harmful Sexual recombination: contributes to most of genetic variation in a population 1. Crossing Over (Meiosis – Prophase I) 2. Independent Assortment of Chromosomes (during meiosis) 3. Random Fertilization (sperm + egg) Hardy-Weinberg Equilibrium Five Fingers of Evolution: Paul Anderson/TED Talks To watch the video use this link: http://ed.ted.com/lessons/five-fingers-of-evolution Population genetics: study of how populations change genetically over time Population: group of individuals that live in the same area and interbreed, producing fertile offspring Gene pool: all of the alleles for all genes in a population Diploid species: 2 alleles for a gene (homozygous/heterozygous) Fixed allele: all members of a population only have 1 allele for a particular trait The more fixed alleles a population has, the LOWER the species’ diversity Hardy-Weinberg Theorm Evolution does NOT occur if the gene pool remains constant (in equilibrium) from one generation to the next. Outside forces must act on a population for there to be change NO Evolution = genetic equilibrium Evolution = NO genetic equilibrium Conditions for Hardy-Weinberg equilibrium 1. 2. 3. 4. 5. No mutations. Random mating. No natural selection. Extremely large population size (no genetic drift) No gene flow (no migration). If at least one of these conditions is NOT met, then the population is EVOLVING! Counting alleles Gene with 2 alleles : B, b frequency of dominant allele (B) = p frequency of recessive allele (b) = q frequencies must = 1 (100%), so: p+q=1 (therefore: 1 – p = q and 1 – q = p) Counting individuals frequency of homozygous dominant: p x p = p2 frequency of homozygous recessive: q x q = q2 frequency of heterozygotes: (p x q) + (q x p) = 2pq frequencies of all individuals must add to 1 (100%), 2 p + 2pq + 2 q =1 Hardy-Weinberg Equilibrium Equation p2 + 2pq + q2 = 1 p + q = 1 (1 – p = q and 1 – q = p) p = dominant allele q = recessive allele p2 = homozygous dominant 2pq = heterozygous q2 = homozygous recessive Strategies for solving H-W Problems: If you are given the genotypes (AA, Aa, aa), calculate p and q by adding up the total # of A and a alleles. 2. If you know phenotypes, then use “aa” to find q2, and then q. (p = 1-q) 3. To find out if population is evolving, calculate p2 + 2pq + q2. If in equilibrium, it should = 1. If it DOES NOT = 1, then the population is evolving! 1. Using Hardy-Weinberg Equation Population: 100 cats 84 black, 16 white How many of each genotype? p2 = .36 BB q2 (bb): 16/100 = .16 q (b): √.16 = 0.4 p (B): 1 - 0.4 = 0.6 2pq = 2(.4)(.6) 2pq = .48 Bb q2 = .16 bb Must assume population in H-W equilibrium! What are the genotypeis frequencies? HARDY WEINBERG PRACTICE One in 1700 US Caucasian newborns have cystic fibrosis. C for normal is dominant c for cystic fibrosis Calculate the allele frequencies for C and c HARDY WEINBERG PRACTICE 1/1700 have cystic fibrosis q2 = 1/1700 q= 0.00059 q = 0.024 p = 1 – 0.024 = 0.976 Frequency of C = 97.6% Frequency of c = 2.4% Now find the genotype frequencies. HARDY WEINBERG PRACTICE cc = q2 = 1/1700 = 0.00059 CC = p2 = (0.976)2 = .953 Cc = 2pq = 2 (0.976) (0.024) = 0.0468 cc = .06% of population CC = 95.3% of population Cc = 4.68% of population Now you can answer questions about the population: How many people in this population are heterozygous? 0.0468 (1700) = 79.5 ~ 80 people are Cc It has been found that a carrier is better able to survive diseases with severe diarrhea. What would happen to the frequency of the "c" if there was a epidemic of cholera or other type of diarrhea producing disease? Cc more likely to survive than CC. c will increase in population The gene for albinism is known to be a recessive allele. In Michigan, 9 people in a sample of 10,000 were found to have albino phenotypes. The other 9,991 had normal skin pigmentation. Assuming hardy-Weinberg equilibrium, what is the allele frequency for the dominant pigmentation allele in this population? q2 = 9/10000 = .0009 q= 0.0009 q = 0.03 p = 1 – 0.03 = 0.97 Frequency of C = 97% Frequency of c = 3% Frequency of CC and Cc CC = p2 = (.97)2 = .9409 Cc = 2pq = 2(.97)(.03) = .0582 CC = 94.09% = 9409 Cc = 2.9% = 582 II. Causes of Evolution . Violations to H-W Equilibrium – cause evolution Minor Causes of Evolution: #1 - Mutations Rare, very small changes in allele frequencies #2 - Nonrandom mating Affect genotypes, but not allele frequencies Major Causes of Evolution: Natural selection, genetic drift, gene flow (#3-5) Major Causes of Evolution #3 – Natural Selection Individuals with variations better suited to environment pass more alleles to next generation Major Causes of Evolution #4 – Genetic Drift Small populations have greater chance of fluctuations in allele frequencies from one generation to another Examples: Founder Effect Bottleneck Effect Genetic Drift Founder Effect A few individuals isolated from larger population Certain alleles under/over represented Polydactyly in Amish population Bottleneck Effect Sudden change in environment drastically reduces population size Northern elephant seals hunted nearly to extinction in California Major Causes of Evolution #5 – Gene Flow Movement of fertile individuals between populations Gain/lose alleles Reduce genetic differences between populations How does natural selection bring about adaptive evolution? Fitness : the contribution an individual makes to the gene pool of the next generation Natural selection can alter frequency distribution of heritable traits in 3 ways: 1. Directional selection 2. Disruptive (diversifying) selection 3. Stabilizing selection Directional Selection: eg. larger black bears survive extreme cold better than small ones Disruptive Selection: eg. small beaks for small seeds; large beaks for large seeds Stabilizing Selection: eg. narrow range of human birth weight Sexual selection Form of natural selection – certain individuals more likely to obtain mates Sexual dimorphism: difference between 2 sexes Size, color, ornamentation, behavior Sexual selection Intrasexual – selection within same sex (eg. M compete with other M) Intersexual – mate choice (eg. F choose showy M) Preserving genetic variation Diploidy: hide recessive alleles that are less favorable Heterozygote advantage: greater fitness than homozygotes eg. Sickle cell disease Natural selection cannot fashion perfect organisms. 1. 2. 3. 4. Selection can act only on existing variations. Evolution is limited by historical constraints. Adaptations are often compromises. Chance, natural selection, and the environment interact.