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Natural Selection and Evolution of Human Population.docx

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McMaster University
Bhagwati Gupta

September 16 , 2013 Biology 2C03: Genetics Natural Selection and Evolution of Human Populations Terminology - Population genetics: branch of genetics that studies the genetic makeup of groups of individuals and how a group’s genetic composition changes over time - Mendelian population: an interbreeding group that share a common set of genes, the “gene pool” - Evolution is changes in a “gene pool”, that is changes in the frequencies of different alleles Population Genetics of β-Globin Alleles S - Last class we learned that the β mutation appears with high frequency in several contemporary populations, particularly African populations. Why is this? - Hypothesis: the β mutation confers a selective advantage for these populations Malaria and Sickle Cell Trait - β is highest in areas where malaria is prevalent Symptoms of Malaria - The general symptoms include (within 1-2 weeks):  Headache  Nausea  High fever  Vomiting  Flu-like symptoms - Further serious complication involving the kidneys and brain can then develop leading to delirium and coma  Especially among young individuals - Malarial infections are chronic and individuals are susceptible to other infections which may in turn lead to death Ecologist Micheal Fay in Congo - Frequently visits congo - Chooses not to take malaria medication frequently as they can lead to liver and kidney damage - Waits until he is extremely sick until he takes the medication Malaria – Life Cycle - Malaria is caused by a protozoan such as Plasmodium falciparum - Carried by a vector, in this case the mosquito, Anopheles gambeii, Plasmodium embryos live in the mosquito - Begin larval development in the mammalian host, transferred through a mosquito bite - Plasmodium larvae mature in the liver of the host animal and alter in the red blood cells  Causes red blood cells to burst Current Efforts to Control Malaria - Mosquito lab: wait for larvae to mature then selectively removing male mosquitoes  Releasing female mosquitoes into the wild in order to have a sterile population - Pesticides: in open water  Open water is ideal breeding grounds for mosquitoes - Traditional remedies: bark from tree - Mosquito nets How Does Malaria Relate to Sickle Cell Disease? - Sickled red blood cells (RBCs) are fragile and the average RBC lifespan in heterozygotes is shorter due to the abnormal haemoglobin - Shorter RBC lifespan interrupts the life cycle of the Plasmodium larvae, preventing proliferation of malaria in the human host - Molecular evidence for selective advantage against malaria - Individuals with β β genotype and β β are more resistant to malaria than those with β β genotype Population Genetics of Hemoglobin - β β : malaria resistance, but still have sickle cell anemia  Die young, often before reproducing (loss of β allele from population) - β β : die from malaria at high frequency (loss of β allele from population) S A - β β : malaria resistance and no sickle cell anemia  Heterozygous advantage (gain of β β in population) - Natural selection maintains both alleles in population and therefore both homozygous phenotypes are maintained (heterozygous advantage) - Balanced polymorphism Allele Frequency - Allele frequency: the frequency of a single allele of a gene within the whole population - Gene pool: all possible alleles - Frequency of blue allele of f(pink) = 30/100 = 30% or 0.3 - Frequency of green allele or f(purple) = 70/100 = 70% or 0.7 - How do we actually assay allele frequency in the gene pool? Calculating Allele Frequency – Step 1 - First we calculate genotypic frequency - A diploid individual carries a pair of alleles  E.g. for A gene: AA or Aa or aa  f(AA) = number of AA individuals/total number  f(Aa) = number of Aa individuals/total number  f(AA) + f(Aa) + f(aa) = 1  Three different calculations in order to define the population Useful Terminology - A diploid individual carries a pair of alleles  E.g. for gene A: AA or Aa or aa - Total number of individuals in a population = N - Total number of alleles in a population = 2N - The number of individuals who are specifically the AA genotype would be n AA - For a locus with only 2 alleles (A or a), the frequencies of alleles are usually represented by symbols p and q Calculating Allele Frequencies - ( ) - ( ) ⁄ - ( ) ( ) ⁄ ( ) - ( ) ⁄ - ( ) ( ) ⁄ ( ) - p + q =1 Individual vs. Population - Remember how we used a Punnett square to determine genotypes of offspring? - Mendel: patterns of inheritance for a cross between two individuals are based upon frequencies of alleles in the individual  E.g. in an individual with the genotype Bb:  p(B) = 0.5  p(b) = 0.5  Results of random union of the two gametes produced by two individuals, each heterozygous for a given trait. As a result of meiosis, half of the gametes produced by each parent will carry allele B; the other half allele b - But, frequencies of two alleles in an entire population are unlikely to be the same  E.g.  f(B) = 0.8  f(b) = 0.2  Results of random union of the gametes produced by an entire population with a gene pool containing 80% B and 20% b Calculating Genotypic and Phenotypic Ratios of Offspring in a Population - B is a dominant allele for dark fur colour in cats - BB and Bb genotypes have black fur - B is the recessive alleles and only bb genotype give brown fur - In a population,80% of gametes carry a B allele; 20% of gametes carry a b allele - Genotypic ratios:  64% of offspring will be BB  32% of offspring will be Bb  4% of offspring will be bb - Phenotypic ratios:  96% black fur  4% brown fur Will Brown Fur Colour Ever Disappear from the Population? - Re-calculate the “new” gene pool of the next generation:  Frequency of B = f(B) =  BB: all gametes are B (100%)  Bb: half of gametes are B (50%)  f(B) = 0.64 + ½(0.32) = 0.80 or 80%  Frequency of b = f(b) =  bb: all gametes are b (100%)  Bb: half of gametes are b (50%)  f(b) = 0.04 + ½(0.32) = 0.20 or 20% - Same allele frequencies maintained - Hardy-Weinberg principle or equilibrium Hardy-Weinberg Principle or Equilibrium - Total number of alleles of a gene in a population is the gene pool. - Here we have two alleles in the gene pool  p = f(B) = frequency of one allele  q = f(b) = frequency of alternate allele 2  p = fraction of population homozygous, BB  q = fraction of population homozygous, bb  2pq = fraction of population heterozygous, Bb  p + q = 1 2 2 2  (p+q) = p + 2pq + q  At equilibrium, the ratio is maintained:  BB:Bb:bb; p :2pq:q 2  Since p+q = 1, p + 2pq + q = 1  We can use this to determine genotype frequencies or allele frequencies  So, recessive alleles are NOT lost (at equilibrium) - As long as certain conditions are met, gene frequencies and genotype ratios in a randomly-breeding population will remain constant from one generation to the next - Populations are able to maintain a reservoir of variability so that if future conditions require it, the gene pool can change Disrupting the Equilibrium - What are these certain conditions? - What forces lead to change in allele frequencies and result in evolutionary change - Equilibrium is maintained given the following assumptions: 1. No mutation 2. No gene flow/migration 3. No genetic drift 4. Random mating 5. No natural selection Mutation - Change in nucleotide base - Mutations can create new alleles or alter allele frequencies - With no mutations, the composition of the gene pool remains the same generation after generation, if the other conditions for Hardy-Weinberg equilibrium are also met - Mutations change the composition of the gene pool. New alleles are introduced and allelic frequencies change - The mutation rate for most genes are low; so change in allelic frequency due to mutation in one generation is very small - Effect on Hardy-Weinberg equilibrium negligible Gene Flow/Migration - Different and isolated populations can have distinct gene pools (alleles and allele frequencies) - However, if individuals from different populations are introduced they can introduce new alleles or alter gene frequencies - Isolation of a population of trees prevents changes in the gene pool due to immigration and emigration - Immigration of alleles in pollen from a neighbouring population of trees can cause a change in the composition of the gene pool - Overall effect migration twofold: 1. Prevents populations from being genetically distinct from one another 2. Increases genetic variation within populations, assuming that there is an introduction of new alleles - Different alleles may arise in different populations due to rare mutational events, these alleles spread to new populations by migration th September 17 , 2013 Genetic Drift - Hardy-Weinberg law assumes random mating in an infinitely large population - Particularly in small populations, chance alone may lead to the loss of individuals, changing allele frequencies - An earthquake that kills three people out of a population of 10 million has little effect on the composition of the gene pool - An earthquake that kills three people out of a band of 20 individuals has significant effect on the composition of the gene pool - When population is limited, the gametes may be biased causing allelic frequencies to change Causes of Genetic Drift - Sampling error  Deviation from expected ratio due to limited sample size  Analogous to flipping a coin 10 times vs. 10,000 times 1. Founder effect  Establishment of a population be a small number of individuals  Horned sheep: o 12 sheep brought to conservation area to start a new population reduced the total alleles available for further generations o Numbers begin to dwindle as they no longer had enough genetic variation to survive in a natural habitat 2. Genetic bottleneck  Population undergoes drastic reduction in population size  Sea lions: o Human hunting reduced the allelic frequency of the population Genetic Bottleneck - Original population - Bottleneck: catastrophic reduction in population - Chance survivors - New population with reduced genetic variation Effects of Genetic Drift 1. Produces changes allelic frequency within a population  Allelic frequency may increase or decrease  Figure 25.13  Fixation of allele A1  Fixation of allele A2  Or peaks and valleys of changes in allelic freuqency 2. Reduce genetic variation  If allele reaches frequency of 1, we say it has reached fixation 3. Different population diverge genetically with time  Different population acquire genetic differences Nonrandom Mating - If individuals exhibit a preference in mate selection, gene frequencies may change (sexual selection) - Random mating: coral polyps disperse their sperm into the ocean currents. Contact with an egg in another coral is completely up to chance - Assortative mating: blister beetles are most likely to mate with partners of the same size Sexual Seletion - “(Sexual selection) depends, not on a struggle for existence, bu
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