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Slide 2 AP Biology 2007-2008 Evolution of Populations Doonesbury - Sunday February 8, 2004 Slide 3 AP Biology Populations evolve Natural selection acts on individuals differential survival survival of the fittest differential reproductive success who bears more offspring Populations evolve genetic makeup of population changes over time favorable traits (greater fitness) become more common Presence of lactate dehydrogenase Mummichog Slide 4 AP Biology 2007-2008 Individuals DONT evolve Individuals survive or dont survive Populations evolve Individuals are selected Individuals reproduce or dont Slide 5 AP Biology Fitness Survival & Reproductive success individuals with one phenotype leave more surviving offspring Body size & egg laying in water striders Slide 6 AP Biology Variation & natural selection Variation is the raw material for natural selection there have to be differences within population some individuals must be more fit than others Slide 7 AP Biology Mean beak depth of parents (mm) Medium ground finch 8 891011 9 10 11 1977198019821984 Dry year Wet year Beak depth Beak depth of offspring (mm) Where does Variation come from? Mutation random changes to DNA errors in mitosis & meiosis environmental damage Sex mixing of alleles recombination of alleles new arrangements in every offspring new combinations = new phenotypes spreads variation offspring inherit traits from parent Slide 8 AP Biology 5 Agents of evolutionary change MutationGene Flow Genetic DriftSelection Non-random mating Slide 9 AP Biology 1. Mutation & Variation Mutation creates variation new mutations are constantly appearing Mutation changes DNA sequence changes amino acid sequence changes proteins: Structure function changes in protein may change phenotype & therefore change fitness Slide 10 AP Biology 2. Gene Flow Movement of individuals & alleles in & out of populations seed & pollen distribution by wind & insect migration of animals sub-populations may have different allele frequencies causes genetic mixing across regions reduce differences between populations Slide 11 AP Biology Human evolution today Gene flow in human populations is increasing today transferring alleles between populations Are we moving towards a blended world? Slide 12 AP Biology 3. Non-random mating Sexual selection Slide 13 AP Biology Warbler finch Tree finches Ground finches 4. Genetic drift Effect of chance events founder effect small group splinters off & starts a new colony bottleneck some factor (disaster) reduces population to small number & then population recovers & expands again Slide 14 AP Biology Founder effect When a new population is started by only a few individuals some rare alleles may be at high frequency; others may be missing skew the gene pool of new population human populations that started from small group of colonists example: colonization of New World Slide 15 AP Biology Distribution of blood types Distribution of the O type blood allele in native populations of the world reflects original settlement Slide 16 AP Biology Distribution of blood types Distribution of the B type blood allele in native populations of the world reflects original migration Slide 17 AP Biology Out of Africa Likely migration paths of humans out of Africa Many patterns of human traits reflect this migration 50,000ya 10-20,000ya Slide 18 AP Biology Bottleneck effect When large population is drastically reduced by a disaster famine, natural disaster, loss of habitat loss of variation by chance event alleles lost from gene pool not due to fitness narrows the gene pool Slide 19 AP Biology Cheetahs All cheetahs share a small number of alleles less than 1% diversity as if all cheetahs are identical twins 2 bottlenecks 10,000 years ago Ice Age last 100 years poaching & loss of habitat Slide 20 AP Biology Conservation issues Bottlenecking is an important concept in conservation biology of endangered species loss of alleles from gene pool reduces variation reduces adaptability Breeding programs must consciously outcross Peregrine Falcon Golden Lion Tamarin Slide 21 AP Biology 5. Natural selection Differential survival & reproduction due to changing environmental conditions climate change food source availability predators, parasites, diseases toxins combinations of alleles that provide fitness increase in the population adaptive evolutionary change Slide 22 AP Biology 5 Agents of evolutionary change MutationGene Flow Genetic DriftSelection Non-random mating Slide 23 AP Biology Directional, Disruptive, and Stabilizing Selection Selection Favors certain genotypes by acting on the phenotypes of certain organisms Three modes of selection are Directional Disruptive Stabilizing Slide 24 AP Biology 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 Slide 25 AP Biology The three modes of selection Fig 23.12 AC (a) Directional selection shifts the overall makeup of the population by favoring variants at one extreme of the distribution. In this case, darker mice are favored because they live among dark rocks and a darker fur color conceals them from predators. (b) Disruptive selection favors variants at both ends of the distribution. These mice have colonized a patchy habitat made up of light and dark rocks, with the result that mice of an intermediate color are at a disadvantage. (c) Stabilizing selection removes extreme variants from the population and preserves intermediate types. If the environment consists of rocks of an intermediate color, both light and dark mice will be selected against. Phenotypes (fur color) Original population Original population Evolved population Frequency of individuals Slide 26 AP Biology The Preservation of Genetic Variation Various mechanisms help to preserve genetic variation in a population Diploidy Maintains genetic variation in the form of hidden recessive alleles Slide 27 AP Biology Heterozygote Advantage Some individuals who are heterozygous at a particular locus Have greater fitness than homozygotes Natural selection Will tend to maintain two or more alleles at that locus Slide 28 AP Biology Neutral Variation Neutral variation Is genetic variation that appears to confer no selective advantage Slide 29 AP Biology Sexual Selection Sexual selection Is natural selection for mating success Can result in sexual dimorphism, marked differences between the sexes in secondary sexual characteristics Slide 30 AP Biology The Evolutionary Enigma of Sexual Reproduction Sexual reproduction Produces fewer reproductive offspring than asexual reproduction, a so-called reproductive handicap Figure 23.16 Asexual reproduction Female Sexual reproduction Female Male Generation 1 Generation 2 Generation 3 Generation 4 Slide 31 AP Biology If sexual reproduction is a handicap, why has it persisted? It produces genetic variation that may aid in disease resistance Slide 32 AP Biology 2005-2006 Any Questions?? Slide 33 AP Biology