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1 1 Genes Within Populations Chapter 16 2 1. The Gene Pool •a. Members of a species can interbreed & produce fertile offspring b. Species have a shared gene pool • 3 3 The Gene Pool •2. Different species do NOT exchange genes by interbreeding Example: Different species that can interbreed =produce sterile or less viable offspring e.g. Mule 4 4 Populations •A group of the same species living in an area No two individuals are exactly alike (variations) More Fit individuals survive & pass on their traits • • 5 5 Speciation Definition: Formation of new species a. One species may split into 2 or more species b. species may evolve into a new species Requires very long periods of time • 6 6 Five Agents of Evolutionary Change Selection pressures: avoiding predators matching climatic condition pesticide resistance 7 7 Modern Evolutionary Thought 8 Modern Synthesis Theory • Combines Darwinian • • selection and Mendelian inheritance Population genetics study of genetic variation within a population Emphasis on quantitative characters 9 9 Modern Synthesis Theory • Today’s theory on evolution • Recognizes that GENES are responsible for the inheritance of characteristics • Recognizes that POPULATIONS, not • individuals, evolve due to natural selection & genetic drift Recognizes that SPECIATION usually is due to the gradual accumulation of small genetic changes 10 10 Describing genetic structure • genotype frequencies • allele frequencies rr = white Rr = pink RR = red 11 11 Genetic variation in space and time Why is genetic variation important? • adaptation to environmental change - conservation •divergence of populations - biodiversity 12 12 variation no variation 13 13 Why is genetic variation important? variation global warming survival EXTINCTION!! no variation 14 14 Why is genetic variation important? divergence variation no variation NO DIVERGENCE!! 15 15 Microevolution • Changes occur in gene pools due to • • • a.mutation, b.natural selection, c.genetic drift. Gene pool changes cause more VARIATION in INDIVIDUALS in the population This process is called MICROEVOLUTION Example: Bacteria becoming unaffected by antibiotics (resistant) 16 16 Species & Populations • Population - a localized group of individuals of the same species. • Species - a group of populations whose individuals have the ability to breed and produce fertile offspring. Gene Pool is defined by TOTAL GENES 17 17 18 18 Gene Pools •A population’s gene pool composed of the total # all genes in the population at any time. If all members of a population are #11. ) homozygous for a particular allele, then the allele is fixed in the gene pool. • • 19 19 The Hardy-Weinberg Theorem •Used to describe a non-evolving population. •Shuffling of alleles by meiosis and random fertilization have no effect on the overall gene pool. Natural populations are NOT expected to actually be in HardyWeinberg equilibrium. • 20 20 Assumptions of the H-W Theorem a. Large population size - small populations have fluctuations in allele frequencies (e.g., fire, storm). b. No migration - immigrants can change the frequency of an allele by bringing in new alleles. c. No net mutations - if alleles change from one to another, this will change the frequency of those alleles. 21 21 Assumptions of the H-W Theorem d. Random mating - if certain traits are more desirable, then individuals with those traits will be selected e. No natural selection . 22 22 Hardy-Weinberg Equilibrium The Hardy-Weinberg Equation: 1.0 = p2 + 2pq + q2 where p2 = frequency of AA genotype; 2pq = frequency of Aa q2 = frequency of aa genotype 23 23 Hardy-Weinberg Equilibrium 24 24 25 25 26 26 Evolution within a species or a population is microevolution. Microevolution refers to changes in allele frequencies in a gene pool and represents a change in a population. 27 27 How does genetic structure change? Causes of Microevolution • a) Genetic Drift • b) Gene flow • c) Natural Selection #18. Changes in allele frequencies • d) Mutations • non-random mating 28 28 1) Genetic drift Genetic drift = the alteration of the gene pool of a small population due to chance. Two factors may cause genetic drift: 29 29 19 Bottleneck effect leads reduces genetic variability following a large disturbance such as an earthquake that removes a large portion of the population. The surviving population often does not represent the Original 30 30 20) Founder effect may lead to reduced variability when a few individuals from a large population colonize an isolated habitat. 31 31 Genetic drift Before: 8 RR 0.50 R 8 rr 0.50 r After: 2 RR 6 rr 0.25 R 0.75 r 32 32 33 33 34 34 Genetic Drift - Bottleneck Effect 35 35 Natural selection Resistance to antibacterial soap Generation 1: 1.00 not resista 0.00 resistant 36 36 Natural selection Resistance to antibacterial soap Generation 1: 1.00 not resista 0.00 resistant 37 37 Natural selection Resistance to antibacterial soap Generation 1: 1.00 not resistant 0.00 resistant Generation 2: 0.96 not resistant 0.04 resistant mutation! 38 38 Natural selection Resistance to antibacterial soap Generation 1: 1.00 not resist 0.00 resistant Generation 2: 0.96 not resist 0.04 resistant Generation 3: 0.76 not resist 0.24 resistant 39 39 Natural selection Resistance to antibacterial soap Generation 1: 1.00 not resist 0.00 resistant Generation 2: 0.96 not resist 0.04 resistant Generation 3: 0.76 not resist 0.24 resistant Generation 4: 0.12 not resist 0.88 resistant 40 40 *Yes, I realize that this is not really a cheetah. 41 41 2) Natural selection As previously stated, differential success in reproduction based on heritable traits results in selected alleles being passed to relatively more offspring (Darwinian inheritance). The only agent that results in adaptation to environment. 3) Gene flow -is genetic exchange due to the migration of fertile individuals or gametes between populations. 42 42 4) Mutation Mutation is a change in an organism’s DNA and is represented by changing alleles. a. Mutations can be transmitted in gametes to offspring, and immediately affect the composition of the gene pool. b. The original source of variation and the driving force for Natural Selection 43 43 44 44 Genetic Variation, the Substrate for Natural Selection Genetic (heritable) variation within and between populations: exists both as what we can see (e.g., eye color) and what we cannot see (e.g., blood type). Not all variation is heritable. Environment also can alter an individual’s phenotype. 45 45 Industrial Melanism of Butterfly Population Industrial Melanism 46 46 Variation between populations Geographic variations are differences between gene pools due to differences in environmental factors. It often occurs when populations are located in different areas, but may also occur in populations with isolated individuals. 47 47 48 48 Mutation and sexual recombination generate genetic variation a. New alleles originate only by mutations (heritable only in gametes; many kinds of mutations; mutations in functional gene products most important). - Mutations are more beneficial (rare) in changing environments. (Example: HIV resistance to antiviral drugs.) b. Sexual recombination is the source of most genetic differences between individuals in a population. 49 49 Diploidy and balanced polymorphism preserve variation a. Diploidy often hides genetic variation from selection in the form of recessive alleles. Dominant alleles “hide” recessive alleles in heterozygotes. b. Balanced polymorphism is the ability of natural selection to maintain stable frequencies of at least two phenotypes. Heterozygote advantage is one example of a balanced polymorphism, where the heterozygote has greater survival and reproductive success than either homozygote (Example: Sickle cell anemia where heterozygotes are resistant to malaria). 50 50 51 51 Diversifying selection 52 52 Sexual selection leads to differences between sexes a. Sexual dimorphism is the difference in appearance between males and females of a species. -Intrasexual selection is the direct competition between members of the same sex for mates of the opposite sex. -This gives rise to males most often having secondary sexual equipment such as antlers that are used in competing for females. -In intersexual selection (mate choice), one sex is choosy when selecting a mate of the opposite sex. -This gives rise to often amazingly sophisticated secondary sexual characteristics; e.g., peacock feathers. 53 53 54 54 55 55 Sickle Cell and Malaria 56 56 Evolutionary Change in Spot Number 57 57 Population genetics – Outline What is population genetics? Calculate - genotype frequencies - allele frequencies Why is genetic variation important? How does genetic structure change? 58 58 Example use of H-W theorem 1000-head sheep flock. No selection for color. Closed to outside breeding. 910 white (B_) 90 black (bb) 59 59 Start with known: f(black) = f(bb) = .09 =q2 q q .09 .3 f (b) 2 Then, p = 1 – q = .7 = f(B) f(BB) = p2 = .49 f(Bb) = 2pq = .42 f(bb) = q2 = .09 60 60 In summary: Allele freq. f(B) = p = .7 (est.) f(b) = q = .3 (est.) Genotypic freq. f(BB) = p2 = .49 (est.) f(Bb) = 2pq = .42 (est.) f(bb) = q2 = .09 (actual) Phenotypic freq. f(white) = .91 (actual) f(black) = .09 (actual) 61 61 Mink example using H-W Group of 2000homo (1920 brown, 80 platinum) in equilibrium. We know f(bb) = 80/2000 = .04 = q2 f(b) = (q2) = .04 = .2 f(B) = p = 1- q = .8 f(BB) = p2 = .64 f(Bb) = 2pq = .32 62 62