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Transcript
Population Genetics & Hardy-Weinberg EQ: How do you determine if a population is evolving or not? What are the components to the Hardy-Weinberg Principle? If and component of H-W is violated, what does this infer? Population Genetics – the study of how populations change genetically over time. Gene pool: The combined genotypes of all the members of a population. Allele frequency: how many specific alleles occur out of the total alleles for a gene in a gene pool. A population is evolving when allele frequencies change over time. Ex. Trait: Hair Texture Phenotypes: Straight, Wavy, Curly (Incomplete Dominance) Genotypes: HSHS, HSHC, HCHC Generation #1 There are 2 straight haired people, 2 wavy haired people, and 2 curly haired people: Allele frequency HS = 3/6 = 50% HC = 3/6 = 50% (3 comes from = 3 HS alleles and 3 HC alleles; 6 comes from there are 6 total alleles in this gene pool) Genotypic and phenotypic frequencies can also be analyzed: Let’s look at phenotypic frequency … Phenotypic ratio: [there are 3 total genotypes (bottom number), each expressing a phenotype; out of the 3 genotypes there is 1 straight, 1 wavy, and 1 curly (top numbers)] Straight = 1/3 Wavy = 1/3 Curly = 1/3 If the phenotypic ratio changes from one generation to the next, it DOES NOT indicate the population is evolving. See Generation #2 1 straight, 4 wavy, 1 curly HSHS, HSHC, HSHC, HSHC, HSHC, (straight) (wavy) Allele frequency HS = 6/12 = 50% HCHC (curly) HC = 6/12 = 50% Phenotypic frequencies Straight = 1/6 Wavy = 4/6 = 2/3 Curly = 1/6 **Population at Hardy-Weinberg equilibrium (or) …is at Genetic Equilibrium (or)…is not evolving. [All mean the same thing] The allele frequencies must change over time to show a population that is evolving. [Different combinations of alleles can create different phenotypes even if the allele frequency doesn’t change. Since the allele frequency didn’t change from generation #1 to generation #2 this population didn’t evolve; so population is said to be at Hardy-Weinberg equilibrium.] See Generation #3 4 straight, 1 wavy, 0 curly HSHS, HSHS, HSHS, HSHS, (Straight) Allele frequency HS = 9/10 HSHC (wavy) HC 1/10 Phenotypic frequencies Straight = 4/5 Wavy = 1/5 Curly = 0/5 = 0 So,….since this population’s allele frequency changed from generation #2 to generation #3, the population has evolved. The Hardy-Weinberg Law = Frequencies of alleles in a given population will remain constant unless acted on by an outside factor. The Hardy-Weinberg Principle suggests reasons why a population would not evolve. 1. Large population – therefore no genetic drift. Genetic drift: random loss of alleles by chance - Alleles frequency is more constant in large populations - drift happens in small populations and have drastic effects 2. No gene flow – no migration - No immigration (migrating into) - No emigration (migrating out of [exit]) 3. No mutations – no introduction of new alleles 4. Random mating – no sexual selection; no selection of a mate due to specific traits. 5. No natural selection – If so, then individual carrying alleles that equip them with genes that express favorable traits will alter the allele frequencies.