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The Evolution of Populations Smallest unit of evolution – microevolution – change in the allele frequencies os a population over generations (ex: dog breeds) Darwin did not know how organisms passed traits to offspring-1866 – Mendel published his paper on genetics-Mendelian genetics supports Darwin’s theory à evolution is based on genetic variation Sources of Genetic Variation:-Point mutations: changes in one base (eg. Sickle cell)-Chromosomal mutations: delete, duplicate, disrupt, rearrange à usually harmful-Sexual recombination: contributes to most of the genetic variation w/i a population 1. crossing over (meiosis – prophase I) 2. Independent assortment of chromosomes (during meiosis) 3. random fertilization (sperm + egg) 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 all the members of the 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 Principle: the allele and genotype frequencies of a population will remain constant from generation to generation …unless they are acted upon by forces other than Mendelian segregation and recombination of alleles-Equilibrium – allele and genotype frequencies remain constant Conditions for Hardy-Weinberg equilibrium:1. no mutations2. random mating3. no natural selection4. extremely large population size5. no gene flowIf at least one of these conditions is NOT met, then the population is EVOLVING. Hardy-Weinberg PrincipleAllele Frequencies:-Gene with 2 alleles: p and q p = frequency of dominant allele (A) q = frequency of recessive allele (a) p + q = 1 NOTE: 1 – p = q and 1 – q = p Hardy-Weinberg EquationGenotype Frequencies:3 Genotypes (AA, Aa, aa) p2 + 2pq + q2 = 1 p2 = AA (homozygous dominant)2pq = Aa (heterozygous)q2 = aa (homozygous recessive) Strategies for solving H-W problems:1. If you are given the genotypes (AA, Aa, aa), calculate p and q by adding up the total # of A and a alleles2. If you know the phenotypes, then use aa to find q2, and then q. (p=1-q)3. Use p2 + 2pq + q2 to find genotype frequencies.4. If p and q are not constant from generation to generation then the POPULATION IS EVOLVING. ***do practice problems*** CAUSES OF EVOLUTION Remember…Conditions for Hardy-Weinberg equilibrium:1. no mutations2. random mating3. no natural selection4. extremely large population size5. no gene flowIf at least one of these conditions is NOT met, then the population is EVOLVING. Minor causes of evolution:1. Mutations – rare, very small changes in allele frequencies2. Nonrandom mating – affect genotypes, but not allele frequenciesMajor causes of evolution: 3-5. Natural selection, genetic drift, gene flow Major causes of evolution#3 – Natural Selection-Individuals with variations better suited to environment pass more alleles to next generation.#4 – Genetic Drift-Small populations have greater chance of fluctuations in allele frequencies from one generation to another.-ex: founder effect – a few individuals isolated from larger population. Certain alleles are under/over represented (ex: polydactyly in Amish population)-ex: bottleneck effect – sudden change in environment drastically reduces population size.#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 (ex: larger black bears survive extreme cold better than small ones) 2. Disruptive (diversifying) selection (ex: small beaks for small seeds, large beaks for large seeds) 3. Stabilizing selection (ex: 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)-Intrasexual – selection within the same sex (eg. males compete w other males)-Intersexual – mate choice (eg. females choose showy males) Preserving Genetic Variation-Diploidy: hide recessive alleles that are less favorable-Heterozygote advantage: greater fitness than homozygotes (ex: sickle cell disease) Natural Selection cannot fashion perfect organisms1. Selection can only act on existing variations2. Evolution is limited by historical constraints3. Adaptations are often compromises4. Chance, natural selection, and the environment interact
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