lecture 2: evolution of populations campbell & reece chapters: chapter 23 microevolution –...
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Lecture 2: Evolution of Populations Campbell & Reece chapters: Chapter 23 Microevolution evolution at the population level = change in allele frequencies over generations Slide 2 Genetics = science dealing with inheritance or heredity, the transmission of acquired traits Slide 3 Ultimate source of heritable variation is change in DNA Change in DNA caused by: 1) Mutation 2) Genetic Recombination Slide 4 Mutations = change in genotype other than by recombination. Three types: 1) Point Mutations 2) Chromosome Mutations 3) Change in Chromosome Number Slide 5 1) Point Mutation Change in a single DNA Nucleotide. Change in a single DNA Nucleotide. Point mutation rate per gene = ~1 in 100,000 gametes. In humans: = 1 mutation/gene x (~25,000 genes) 100,000 gametes =~0.25 point mutations/gamete Slide 6 E.g., human hemoglobin: 2 alpha chains (141 amino acids) 2 beta chains (146 amino acids) 1973 sampling of population (thousands): 169 mutation types recorded: 62 substitutions in alpha 99 substitutions in beta 1 deletion in alpha 7 deletions in beta 1 in 2,000 people have mutant hemoglobin gene. hemoglobin Slide 7 2) Chromosome Mutations Rearrangements (including losses and gains) of large pieces of DNA. E.g., inversion: Re-attaches here and here A B C D E F G A B F E D C G [3% of pop. of Edinburgh, Scotland have inversion in Chromosome #1] [Humans differ from chimps by 6 inversions, from gorillas by 8 (also difference in chromosome number)] Slide 8 3) Change in Chromosome No. a) Aneuploidy - change in chromosome number of less than an entire genome. Horse (2n = 64) versus donkey (2n = 62) Humans (2n = 46) versus chimp or gorilla (2n = 48) Some Genetic Diseases Trisomy (addition of a chromosome to the original diploid pair) of chromosome 21 in humans = Down's syndrome. Extra or one sex chromosomes ( e. g., XYY, XXY, X). Slide 9 b) Polyploidy Evolution of chromosome number which is a multiple of some ancestral set. Has been a major mechanism of evolution in plants. Slide 10 Two ways polyploidy can occur: Slide 11 Polyploid evolution of wheat Slide 12 Genetic Recombination (in sexual reproduction) = Natural, shuffling of existing genes, occurring with meiosis and sexual reproduction Two types: Independent Assortment Crossing over Slide 13 Independent assortment Sorting of homologous chromosomes independently of one another during meiosis E. g., (where A,B,&C genes are unlinked) AaBBcc X AabbCC ---> AaBbCc (one of many possibilities) Slide 14 Results in great variation of gametes, and therefore progeny. [E. g., one human: 2 23 = 8,388,608 possible types of gametes (each with different combination of alleles).] Independent assortment Slide 15 Crossing over Exchange of chromatid segments of two adjacent homologous chromosomes during meiosis (prophase). Greatly increases variability of gametes and, therefore, of progeny. Slide 16 Genetic Variation Genetic recombination - source of most variation (in sexual organisms), via new allele combinations. Mutation - ultimate source of variation, source of new alleles and genes. Slide 17 Fitness = measure of the relative contribution of a given genotype to the next generation Can measure for individual or population. Slide 18 Fitness = allele/genotype freq. in future generation allele/genotype freq. in prev. generation E. g., 1st gen. 25%AA : 50%Aa : 25%aa [freq. A = 25% +.5(50%) = 50%] 2nd gen.: 36%AA : 48%Aa : 16%aa [freq. A = 36% +.5(48%) = 60%] Fitness of A allele is 60/50 = 1.2; a is 40/50 = 0.8 Fitness of AA genotype is 36/25 = 1.44, etc. Slide 19 Hardy-Weinberg Equilibrium (1908) The frequency of a gene / allele does not change over time (given certain conditions). A,a = alleles of one gene, combine as AA, Aa, or aa Generation 1: p = freq. A q = freq. a p + q = 1 (100%) pAqa pAp 2 AApqAa qapqAaq 2 aa } =gene frequencies in generation 1 p 2 AA + 2pqAa + q 2 aa = 1 Slide 20 Hardy-Weinberg Equilibrium (1908) Example: Generation 1: p = 0.4 q = 0.6 p + q = 1 (100%) 0.4A0.6a 0.4A0.16AA0.24Aa 0.6a0.24Aa0.36aa } =gene frequencies in generation 1 p 2 AA + 2pqAa + q 2 aa = 0.16 + 0.48 + 0.36 = 1 Slide 21 Hardy-Weinberg Equilibrium (1908) The frequency of a gene / allele does not change over time (given certain conditions). What will be the frequency of alleles in the second generation? p 2 AA + 2pqAa + q 2 aa = 1 freq. A (generation 2) = (p 2 + pq) / (p 2 + 2pq + q 2 ) = p(p + q) / (p + q) 2 = p / (p + q) = p Therefore, freq. A = p; freq. a = q, same as in generation 1. } =gene frequencies in generation 1 Slide 22 Hardy-Weinberg Equilibrium Maintained only if: 1) No mutation Mutations rare, but do occur (1 new mutation in 10,000 - 1,000,000 genes per individual per generation) Slide 23 2) No migration (no gene flow into or out of population) But, can occur... Hardy-Weinberg Equilibrium Slide 24 3) Population size large Two things can disrupt: a) Population bottleneck (large pop. gets very small) b) Founder effect (one or a few individuals dispersed from a large pop.) Hardy-Weinberg Equilibrium Slide 25 4) Mating is random But, most animals mate selectively, e.g., 1) harem breeding (e. g., elephant seals); 2) assortative mating (like mates with like) 3) sexual selection Hardy-Weinberg Equilibrium Slide 26 5) All genotypes equally adaptive (i.e., no selection) But, selection does occur... Hardy-Weinberg Equilibrium Slide 27 If any conditions of Hardy-Weinberg not met: Genotype frequencies change Evolution occurs! Evolution = change in gene frequency of a population over time. Slide 28 Selective Pressure = agent or causative force that results in selection. E. g., for dark skin, selective pressure = UV radiation (UV increases sunburn and skin cancer in lighter skinned individuals) E. g., for light skin, selective pressure = Vitamin D synthesis Slide 29 Genetic Drift = change in genotype solely by chance effects random! promoted by: Population Bottleneck -drastic reduction in population size Founder Effect - isolated colonies founded by small no. individuals Slide 30 Fig. 23-9 Original population Bottlenecking event Surviving population Population Bottleneck Fig. 23-10 Range of greater prairie chicken Pre-bottleneck (Illinois, 1820) Post-bottleneck (Illinois, 1993) (a) Slide 31 Summary: Evolution can occur by two major mechanisms: Natural Selection (non-random) Genetic Drift (random) Slide 32 Pepper Moth: Biston betularia Selective pressure=predation by birds Single gene: AA/Aa = dark aa = light Camoflague selected for! Slide 33 Result: Balanced polymorphism E.g., Sickle Cell Anemia: Mutation = single amino acid subst. in beta chain of hemoglobin --> single a.a. difference. Sickle blood cells Normal blood cells Slide 34 Homozygotes for sickle mutation (HsHs): lethal Sickle Cell Anemia Slide 35 Heterozygotes (HsHn): resistant to malaria, selected for in malaria- infested regions, selected against where malaria not present. Slide 36 General Principle: Selection dependent on the environment! If environmental conditions change, selective pressure can change!! Slide 37 Stabilizing selection - selection against the two extremes in a population (e.g., birth weight in humans, clutch size in birds) Slide 38 Directional selection - selection for one extreme in a population, against the other extreme (e.g., pesticide resistance in insects antibiotic resistance in bacteria) Slide 39 Disruptive selection - selection for the two extremes in a population, against the average forms (e.g., limpets w/ 2 color forms: light & dark in mosaic environment; flies on two hosts: apple & hawthorn) Slide 40 Sexual Selection - selection resulting in greater reproductive fitness in certain individuals of one sex Slide 41 Sexual Selection Intrasexual selection within one sex; competition between members of one sex (usually males) Slide 42 Sexual Selection Intersexual selection between two sexes; preference by one sex for features of the other sex. Usu. female choice. Slide 43 Sexual Selection Slide 44 Balance between survivorship (decreased) reproductive potential (increased) Slide 45 Sexual Selection: decreased survivorship