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POPULATION GENETICS OVERVIEW: THE SMALLEST UNIT OF EVOLUTION Natural selection acts on individuals only populations evolve On phenotypes Genetic variations contribute to evolution Microevolution change in the genetic makeup of a population from generation to generation THE MODERN SYNTHESIS Population genetics study of how populations change genetically over time integrates Mendelian genetics with the Darwinian theory of evolution by natural selection GENE POOLS AND ALLELE FREQUENCIES • gene pool is the total aggregate of genes in a population at any one time gene pool consists of all gene loci in all individuals of the population MAP AREA CANADA ALASKA LE 23-3 Beaufort Sea Porcupine herd range Fairbanks Fortymile herd range Whitehorse THE HARDY-WEINBERG THEOREM describes a population that is not evolving frequencies of alleles and genotypes in a population’s gene pool remain constant from generation to generation provided that only Mendelian segregation and recombination of alleles are at work Mendelian population inheritance preserves genetic variation in a LE 23-4 Generation 1 X CRCR genotype Generation 2 Plants mate CWCW genotype All CRCW (all pink flowers) 50% CW gametes 50% CR gametes come together at random Generation 3 25% CRCR 50% CRCW 50% CR gametes 25% CWCW 50% CW gametes come together at random Generation 4 25% CRCR 50% CRCW 25% CWCW Alleles segregate, and subsequent generations also have three types of flowers in the same proportions If p and q represent the relative frequencies of the only two possible alleles in a population at a particular locus, then p2 + 2pq + q2 = 1 p2 and q2 represent the frequencies of the homozygous genotypes 2pq represents the frequency of the heterozygous genotype p +q=1 p represents the frequency of one allele (dominant) q represents the frequency of the other allele (recessive) The five conditions for non-evolving populations are rarely met in nature: Extremely large population size No gene flow No mutations Random mating No natural selection POPULATION GENETICS AND HUMAN HEALTH We can use the Hardy-Weinberg equation to estimate the percentage of the human population carrying the allele for an inherited disease VARIATION THAT MAKES EVOLUTION POSSIBLE Mutation changes in the nucleotide sequence of DNA cause new genes and alleles to arise point mutation is a change in one base in a gene Chromosomal mutations that delete, disrupt, or rearrange many loci are typically harmful Gene duplication is nearly always harmful Mutation rates are low in animals and plants usually harmless but may have significant impact on phenotype one mutation in every 100,000 genes per generation Mutations are more rapid in microorganisms sexual recombination far more important than mutation in producing the genetic differences that make adaptation possible Deletion Duplication Inversion Translocation A POPULATION’S GENETIC COMPOSITION Three major factors alter allele frequencies and bring about most evolutionary change: Natural selection Genetic drift Gene flow LE 23-7 CWCW CRCR CRCR CRCW Only 5 of 10 plants leave offspring CRCR CWCW CRCW CWCW CRCR CRCW CRCW CRCR CRCR CRCR CRCW CRCW Generation 1 p (frequency of CR) = 0.7 q (frequency of CW) = 0.3 CWCW CRCR Only 2 of 10 plants leave offspring CRCR CRCR CRCR CRCR CRCR CRCR CRCR CRCR CRCW CRCW Generation 2 p = 0.5 q = 0.5 CRCR CRCR Generation 3 p = 1.0 q = 0.0 LE 23-8 Original population Bottlenecking event Surviving population GENETIC VARIATION Genetic variation occurs in individuals in populations of all species It is not always heritable LE 23-9 Map butterflies that emerge in spring: orange and brown Map butterflies that emerge in late summer: black and white POLYMORPHISM Phenotypic polymorphism a population in which two or more distinct morphs for a character are represented in high enough frequencies to be readily noticeable Most species exhibit geographic variation differences between gene pools of separate populations or population subgroups LE 23-11 Heights of yarrow plants grown in common garden Mean height (cm) 100 50 0 3,000 2,000 1,000 Sierra Nevada Range 0 Seed collection sites Great Basin Plateau SELECTION Selection favors certain genotypes by acting on the phenotypes of certain organisms Three modes of selection: Directional Disruptive Stabilizing Directional selection favors individuals at one end of the phenotypic range Disruptive selection favors individuals at both extremes of the phenotypic range Stabilizing selection favors intermediate variants and acts against extreme phenotypes Frequency of individuals LE 23-12 Original population Evolved population Directional selection Original population Phenotypes (fur color) Disruptive selection Stabilizing selection THE PRESERVATION OF GENETIC VARIATION Diploidy maintains genetic variation in the form of hidden recessive alleles Balancing selection occurs when natural selection maintains stable frequencies of two or more phenotypic forms in a population leads to a state called balanced polymorphism Heterozygote Advantage maintains two or more alleles at that locus Ex. sickle-cell allele causes mutations in hemoglobin also confers malaria resistance LE 23-13 Frequencies of the sickle-cell allele 0–2.5% 2.5–5.0% 5.0–7.5% Distribution of malaria caused by Plasmodium falciparum (a protozoan) 7.5–10.0% 10.0–12.5% >12.5% In frequency-dependent selection fitness of any morph declines if it becomes too common in the population LE 23-14 On pecking a moth image the blue jay receives a food reward. If the bird does not detect a moth on either screen, it pecks the green circle to continue a new set of images (a new feeding opportunity). Parental population sample 0.6 Phenotypic variation Experimental group sample 0.5 0.4 Frequencyindependent control 0.3 0.2 0 Plain background Patterned background 20 40 60 Generation number 80 100 SEXUAL SELECTION Sexual selection natural selection for mating success can result in sexual dimorphism marked differences between the sexes in secondary sexual characteristics Intrasexual selection competition among individuals of one sex for mates of the opposite sex Intersexual selection occurs when individuals of one sex (usually females) are choosy in selecting their mates from individuals of the other sex Selection may depend on the showiness of the male’s appearance